feat(contracts): lifecycle — Lock/Start/Pause/Complete/Unlock + queue schedulercontract-lifecycle

Adds the contract lifecycle layer on top of the unified-contracts
backbone (#128). State machine:

    draft → queued → active → shipped → archived

At most one contract per directive sits in `active` at any time —
the queue is serialised because each directive owns a single shared
worktree. Repository helpers handle the transition checks AND
auto-promote the next-up `queued` contract whenever the active slot
frees (pause / complete / unlock-from-active / archive-from-active).

Endpoints (all under /api/v1/contracts/{id}):
  POST /start    draft → queued | active (depending on slot)
  POST /pause    active → queued; promotes next queued
  POST /complete active → shipped; optional pr_url + pr_branch
  POST /unlock   queued | active → draft; promotes if was active

Frontend wiring:
  * `DirectiveContractStatus` now includes `queued`.
  * Migration adds `queued` to the CHECK constraint on
    directive_documents.status.
  * `ContractHeader` component renders breadcrumb + status pill +
    status-driven action buttons + a merge-mode (shared / own_pr)
    radio. Merge mode is editable only while draft / queued so a
    running flow's branch target can't change mid-stream.
  * RepositoryError gains a `Validation(String)` arm; the three
    existing exhaustive matches (files, mesh, versions) get a
    400 BAD_REQUEST response for it.

Drag-to-reorder UI deferred to a small follow-up — the backend
endpoint already exists from the backbone PR.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

10 files changed, 841 insertions, 36 deletions

diff --git a/makima/frontend/src/lib/api.ts b/makima/frontend/src/lib/api.ts
index 17cf1d0..f777ba0 100644
--- a/makima/frontend/src/lib/api.ts
+++ b/makima/frontend/src/lib/api.ts
@@ -3597,7 +3597,7 @@ export async function pickUpOrders(directiveId: string): Promise<PickUpOrdersRes
 // to plain `Contract`. URLs and the user-facing UI already say "contract".
 // =============================================================================
 
-export type DirectiveContractStatus = "draft" | "active" | "shipped" | "archived";
+export type DirectiveContractStatus = "draft" | "queued" | "active" | "shipped" | "archived";
 
 /** How a contract's commits land. `shared` (default): on the directive's
  *  branch — multiple contracts feed one PR. `own_pr`: a contract-specific
@@ -3700,6 +3700,58 @@ export async function reorderDirectiveContract(
   return res.json();
 }
 
+// ----- Lifecycle transitions -----------------------------------------------
+//
+// Lock & Start: draft → queued (if a sibling is active) or active.
+// Pause: active → queued; next queued sibling auto-promotes.
+// Complete: active → shipped, optionally records pr_url + pr_branch.
+// Unlock: queued or active → draft (reactivates editing).
+//
+// All four reach the same /contracts/{id}/<action> endpoint and surface
+// 400 with a plain message string when the transition is invalid.
+
+async function postContractAction(
+  contractId: string,
+  action: "start" | "pause" | "complete" | "unlock",
+  body?: object,
+): Promise<DirectiveContract> {
+  const res = await authFetch(`${API_BASE}/api/v1/contracts/${contractId}/${action}`, {
+    method: "POST",
+    headers: { "Content-Type": "application/json" },
+    body: JSON.stringify(body ?? {}),
+  });
+  if (!res.ok) {
+    let detail = res.statusText;
+    try {
+      const err = await res.json();
+      if (err?.message) detail = err.message;
+    } catch {
+      /* fall through to statusText */
+    }
+    throw new Error(`Failed to ${action} contract: ${detail}`);
+  }
+  return res.json();
+}
+
+export async function startDirectiveContract(contractId: string): Promise<DirectiveContract> {
+  return postContractAction(contractId, "start");
+}
+
+export async function pauseDirectiveContract(contractId: string): Promise<DirectiveContract> {
+  return postContractAction(contractId, "pause");
+}
+
+export async function completeDirectiveContract(
+  contractId: string,
+  opts: { prUrl?: string; prBranch?: string } = {},
+): Promise<DirectiveContract> {
+  return postContractAction(contractId, "complete", opts);
+}
+
+export async function unlockDirectiveContract(contractId: string): Promise<DirectiveContract> {
+  return postContractAction(contractId, "unlock");
+}
+
 /** Steps and tasks attached to a single contract. Drives the per-contract
  *  `tasks/` subfolder in the sidebar — when the contract ships, its
  *  tasks visually move with it. */
diff --git a/makima/frontend/src/routes/document-directives.tsx b/makima/frontend/src/routes/document-directives.tsx
index d589dac..044c8af 100644
--- a/makima/frontend/src/routes/document-directives.tsx
+++ b/makima/frontend/src/routes/document-directives.tsx
@@ -12,11 +12,16 @@ import {
   type DirectiveStep,
   type Task,
   type DirectiveContractTasksResponse as ContractTasksResponse,
+  type DirectiveContractMergeMode as ContractMergeMode,
   listDirectiveContracts as listContracts,
   createDirectiveContract as createContract,
   getDirectiveContract as getContract,
   updateDirectiveContract as updateContract,
   listDirectiveContractTasks as listContractTasks,
+  startDirectiveContract,
+  pauseDirectiveContract,
+  completeDirectiveContract,
+  unlockDirectiveContract,
   createDirectiveTask,
   startDirective,
   pauseDirective,
@@ -41,10 +46,13 @@ const STATUS_DOT: Record<DirectiveStatus, string> = {
   archived: "bg-[#3a4a6a]",
 };
 
-// Per-document status palette. Active/draft documents use the same bright
-// green-ish accent as a running directive; shipped/archived use a muted blue.
+// Per-contract status palette. Active = bright green (currently driving
+// daemons); queued = amber (locked, waiting for the active slot); draft
+// = grey (editable spec); shipped = muted blue (work done); archived =
+// faint navy.
 const DOC_STATUS_DOT: Record<ContractStatus, string> = {
   draft: "bg-[#556677]",
+  queued: "bg-amber-400",
   active: "bg-green-400",
   shipped: "bg-[#75aafc]",
   archived: "bg-[#3a4a6a]",
@@ -892,6 +900,226 @@ function DocumentSidebar({
 }
 
 // =============================================================================
+// Contract header — breadcrumb + status badge + lifecycle action buttons +
+// merge mode radio. Renders above the spec editor in the document path.
+//
+// Action visibility is status-driven:
+//   * draft     → Lock & Start
+//   * queued    → Unlock (back to draft); shows "queued" pill
+//   * active    → Pause, Complete, Unlock; shows "active" + pulsing dot
+//   * shipped   → reopen via spec edit (no buttons here; backend reactivates)
+//   * archived  → no buttons
+//
+// Merge mode (shared / own_pr) is editable while the contract is in
+// `draft` or `queued` — once active, the queue scheduler has already
+// claimed the slot, so flipping the toggle would silently change a
+// running flow's branch target. Locked rows show the value as readonly.
+// =============================================================================
+
+interface ContractHeaderProps {
+  directive: { id: string; title: string; orchestratorTaskId: string | null };
+  doc: Contract;
+  docTitle: string;
+  /** Called with the server's response after any status / merge-mode
+   *  transition so the parent can refresh the editor + sidebar. */
+  onContractChanged: (updated: Contract) => void;
+}
+
+function ContractHeader({
+  directive,
+  doc,
+  docTitle,
+  onContractChanged,
+}: ContractHeaderProps) {
+  const [busy, setBusy] = useState<null | "start" | "pause" | "complete" | "unlock" | "merge_mode">(
+    null,
+  );
+  const [error, setError] = useState<string | null>(null);
+
+  const wrap = useCallback(
+    async (tag: typeof busy, op: () => Promise<Contract>) => {
+      try {
+        setBusy(tag);
+        setError(null);
+        const updated = await op();
+        onContractChanged(updated);
+      } catch (e) {
+        setError(e instanceof Error ? e.message : "Unknown error");
+      } finally {
+        setBusy(null);
+      }
+    },
+    [onContractChanged],
+  );
+
+  const onStart = useCallback(
+    () => wrap("start", () => startDirectiveContract(doc.id)),
+    [doc.id, wrap],
+  );
+  const onPause = useCallback(
+    () => wrap("pause", () => pauseDirectiveContract(doc.id)),
+    [doc.id, wrap],
+  );
+  const onComplete = useCallback(
+    () => wrap("complete", () => completeDirectiveContract(doc.id)),
+    [doc.id, wrap],
+  );
+  const onUnlock = useCallback(
+    () => wrap("unlock", () => unlockDirectiveContract(doc.id)),
+    [doc.id, wrap],
+  );
+  const onMergeMode = useCallback(
+    (mode: ContractMergeMode) =>
+      wrap("merge_mode", () => updateContract(doc.id, { mergeMode: mode })),
+    [doc.id, wrap],
+  );
+
+  const editableMergeMode = doc.status === "draft" || doc.status === "queued";
+
+  return (
+    <div className="px-6 py-3 border-b border-dashed border-[rgba(117,170,252,0.2)] flex flex-col gap-2">
+      {/* Row 1: breadcrumb + status pill + orchestrator indicator */}
+      <div className="flex items-center gap-2 text-[10px] font-mono uppercase tracking-wide text-[#7788aa]">
+        <FileIcon />
+        <span>directives /</span>
+        <span className="text-[#9bc3ff]">
+          {directive.title.trim().length > 0
+            ? directive.title
+            : directive.id.slice(0, 8)}
+        </span>
+        <span>/</span>
+        <span className="text-white">{docTitle}</span>
+        <ContractStatusPill status={doc.status} />
+        {!!directive.orchestratorTaskId && (
+          <span className="ml-auto inline-flex items-center gap-1 text-yellow-400">
+            <span className="inline-block w-1.5 h-1.5 rounded-full bg-yellow-400 animate-pulse" />
+            orchestrator running
+          </span>
+        )}
+      </div>
+
+      {/* Row 2: action buttons (status-driven) + merge mode + error */}
+      <div className="flex items-center gap-2 text-[11px] font-mono">
+        {doc.status === "draft" && (
+          <ContractActionButton onClick={onStart} disabled={busy !== null} variant="primary">
+            {busy === "start" ? "Starting…" : "Lock & Start"}
+          </ContractActionButton>
+        )}
+        {doc.status === "queued" && (
+          <ContractActionButton onClick={onUnlock} disabled={busy !== null}>
+            {busy === "unlock" ? "Unlocking…" : "Unlock"}
+          </ContractActionButton>
+        )}
+        {doc.status === "active" && (
+          <>
+            <ContractActionButton onClick={onPause} disabled={busy !== null}>
+              {busy === "pause" ? "Pausing…" : "Pause"}
+            </ContractActionButton>
+            <ContractActionButton onClick={onComplete} disabled={busy !== null} variant="primary">
+              {busy === "complete" ? "Completing…" : "Mark complete"}
+            </ContractActionButton>
+            <ContractActionButton onClick={onUnlock} disabled={busy !== null}>
+              {busy === "unlock" ? "Unlocking…" : "Unlock"}
+            </ContractActionButton>
+          </>
+        )}
+
+        {/* Merge mode radios — visible always, editable only in draft/queued */}
+        <div className="ml-auto flex items-center gap-2 text-[#7788aa]">
+          <span className="uppercase tracking-wide">merge:</span>
+          <MergeModeRadio
+            value={doc.mergeMode}
+            onChange={onMergeMode}
+            disabled={!editableMergeMode || busy !== null}
+          />
+        </div>
+      </div>
+
+      {error && (
+        <div className="text-[10px] font-mono text-red-400">{error}</div>
+      )}
+    </div>
+  );
+}
+
+function ContractStatusPill({ status }: { status: ContractStatus }) {
+  const styles: Record<ContractStatus, { label: string; cls: string }> = {
+    draft: { label: "draft", cls: "text-[#556677]" },
+    queued: { label: "queued", cls: "text-amber-400" },
+    active: { label: "active", cls: "text-green-400" },
+    shipped: { label: "shipped", cls: "text-[#75aafc]" },
+    archived: { label: "archived", cls: "text-[#7788aa]" },
+  };
+  const s = styles[status];
+  return <span className={`ml-2 normal-case ${s.cls}`}>{s.label}</span>;
+}
+
+function ContractActionButton({
+  children,
+  onClick,
+  disabled,
+  variant,
+}: {
+  children: React.ReactNode;
+  onClick: () => void;
+  disabled?: boolean;
+  variant?: "primary";
+}) {
+  const base =
+    "px-2 py-1 border border-[rgba(117,170,252,0.3)] rounded text-[10px] uppercase tracking-wide transition-colors";
+  const colors =
+    variant === "primary"
+      ? "text-green-300 hover:bg-[rgba(120,200,140,0.1)]"
+      : "text-[#9bc3ff] hover:bg-[rgba(117,170,252,0.1)]";
+  const dim = disabled ? "opacity-50 cursor-not-allowed" : "cursor-pointer";
+  return (
+    <button
+      type="button"
+      onClick={onClick}
+      disabled={disabled}
+      className={`${base} ${colors} ${dim}`}
+    >
+      {children}
+    </button>
+  );
+}
+
+function MergeModeRadio({
+  value,
+  onChange,
+  disabled,
+}: {
+  value: ContractMergeMode;
+  onChange: (mode: ContractMergeMode) => void;
+  disabled?: boolean;
+}) {
+  const opt = (mode: ContractMergeMode, label: string) => {
+    const selected = value === mode;
+    const cls = selected
+      ? "text-white border-[rgba(117,170,252,0.6)] bg-[rgba(117,170,252,0.1)]"
+      : "text-[#7788aa] border-transparent hover:text-[#9bc3ff]";
+    const dim = disabled ? "opacity-50 cursor-not-allowed" : "cursor-pointer";
+    return (
+      <button
+        key={mode}
+        type="button"
+        onClick={() => !disabled && !selected && onChange(mode)}
+        disabled={disabled}
+        className={`px-2 py-0.5 rounded border ${cls} ${dim} text-[10px] uppercase tracking-wide`}
+      >
+        {label}
+      </button>
+    );
+  };
+  return (
+    <div className="flex items-center gap-1">
+      {opt("shared", "shared")}
+      {opt("own_pr", "own pr")}
+    </div>
+  );
+}
+
+// =============================================================================
 // Editor shell — wraps DocumentEditor and handles the "no document selected"
 // and loading states. Two modes:
 //   1) documentId selected → fetch the Contract and edit doc.body via
@@ -1093,35 +1321,15 @@ function EditorShell({
 
     return (
       <div className="flex-1 flex flex-col h-full overflow-hidden">
-        {/* Breadcrumb — directives / <directive title> / <document title>.md */}
-        <div className="px-6 py-3 border-b border-dashed border-[rgba(117,170,252,0.2)]">
-          <div className="flex items-center gap-2 text-[10px] font-mono uppercase tracking-wide text-[#7788aa]">
-            <FileIcon />
-            <span>directives /</span>
-            <span className="text-[#9bc3ff]">
-              {directive.title.trim().length > 0
-                ? directive.title
-                : directive.id.slice(0, 8)}
-            </span>
-            <span>/</span>
-            <span className="text-white">{docTitle}</span>
-            {doc.status === "shipped" && (
-              <span className="ml-2 text-[#75aafc] normal-case">shipped</span>
-            )}
-            {doc.status === "archived" && (
-              <span className="ml-2 text-[#7788aa] normal-case">archived</span>
-            )}
-            {doc.status === "draft" && (
-              <span className="ml-2 text-[#556677] normal-case">draft</span>
-            )}
-            {!!directive.orchestratorTaskId && (
-              <span className="ml-auto inline-flex items-center gap-1 text-yellow-400">
-                <span className="inline-block w-1.5 h-1.5 rounded-full bg-yellow-400 animate-pulse" />
-                orchestrator running
-              </span>
-            )}
-          </div>
-        </div>
+        <ContractHeader
+          directive={directive}
+          doc={doc}
+          docTitle={docTitle}
+          onContractChanged={(updated) => {
+            setDoc(updated);
+            onDocumentChanged();
+          }}
+        />
 
         <DocumentEditor
           // Keying by document id ensures the Lexical editor remounts cleanly
diff --git a/makima/migrations/20260509000000_contract_lifecycle_states.sql b/makima/migrations/20260509000000_contract_lifecycle_states.sql
new file mode 100644
index 0000000..fc9c7bf
--- /dev/null
+++ b/makima/migrations/20260509000000_contract_lifecycle_states.sql
@@ -0,0 +1,19 @@
+-- Add 'queued' to the contract status enum.
+--
+-- The unified directive workflow runs contracts sequentially in the
+-- directive's shared worktree — only one contract is `active` at a time.
+-- When a user clicks "Lock & Start" on a draft, it goes to `active` if
+-- the slot is free, otherwise it goes to `queued` and waits for the
+-- current active contract to ship/archive. The `complete_contract`
+-- handler auto-promotes the lowest-position `queued` row to `active`.
+--
+-- The constraint replacement is straightforward — drop + re-add. No
+-- existing rows can be in 'queued' yet, so the new CHECK is satisfied
+-- by every row.
+
+ALTER TABLE directive_documents
+    DROP CONSTRAINT IF EXISTS directive_documents_status_check;
+
+ALTER TABLE directive_documents
+    ADD CONSTRAINT directive_documents_status_check
+    CHECK (status IN ('draft', 'queued', 'active', 'shipped', 'archived'));
diff --git a/makima/src/db/repository.rs b/makima/src/db/repository.rs
index 12d5e4d..e58f58c 100644
--- a/makima/src/db/repository.rs
+++ b/makima/src/db/repository.rs
@@ -35,6 +35,8 @@ pub enum RepositoryError {
         /// The actual current version in the database
         actual: i32,
     },
+    /// Caller-facing precondition failure (wrong status, etc.).
+    Validation(String),
 }
 
 impl From<sqlx::Error> for RepositoryError {
@@ -54,6 +56,7 @@ impl std::fmt::Display for RepositoryError {
                     expected, actual
                 )
             }
+            RepositoryError::Validation(msg) => write!(f, "Validation error: {}", msg),
         }
     }
 }
@@ -6038,11 +6041,16 @@ pub async fn mark_directive_document_shipped(
 /// Archive a directive document. Sets status = 'archived' and stamps
 /// archived_at = NOW(). Idempotent — archiving an already-archived doc
 /// re-stamps archived_at and bumps version.
+///
+/// If the archived contract was `active`, the next-up `queued` contract
+/// in the same directive auto-promotes to `active` (sequential queue).
 pub async fn archive_directive_document(
     pool: &PgPool,
     document_id: Uuid,
 ) -> Result<Option<DirectiveDocument>, sqlx::Error> {
-    sqlx::query_as::<_, DirectiveDocument>(
+    let mut tx = pool.begin().await?;
+
+    let archived = sqlx::query_as::<_, DirectiveDocument>(
         r#"
         UPDATE directive_documents
         SET status = 'archived',
@@ -6054,8 +6062,270 @@ pub async fn archive_directive_document(
         "#,
     )
     .bind(document_id)
-    .fetch_optional(pool)
-    .await
+    .fetch_optional(&mut *tx)
+    .await?;
+
+    if let Some(ref doc) = archived {
+        promote_next_queued_contract(&mut tx, doc.directive_id).await?;
+    }
+
+    tx.commit().await?;
+    Ok(archived)
+}
+
+// ============================================================================
+// Lifecycle transitions: start / pause / complete / unlock
+//
+// The lifecycle is `draft → queued → active → shipped → archived`. At most
+// one contract per directive sits in `active` at a time — the queue is
+// serialised because a directive owns a single shared worktree. Helpers
+// below enforce that invariant in SQL transactions.
+// ============================================================================
+
+/// Lock a draft contract and either activate it (if no sibling is active)
+/// or queue it. Returns the updated row, or `Ok(None)` if the contract
+/// doesn't exist. Errors with `RepositoryError::Validation` if the
+/// contract is in any state other than `draft`.
+pub async fn start_contract(
+    pool: &PgPool,
+    contract_id: Uuid,
+) -> Result<Option<DirectiveDocument>, RepositoryError> {
+    let mut tx = pool.begin().await?;
+
+    let current = sqlx::query_as::<_, DirectiveDocument>(
+        r#"SELECT * FROM directive_documents WHERE id = $1"#,
+    )
+    .bind(contract_id)
+    .fetch_optional(&mut *tx)
+    .await?;
+
+    let current = match current {
+        Some(c) => c,
+        None => return Ok(None),
+    };
+
+    if current.status != "draft" {
+        return Err(RepositoryError::Validation(format!(
+            "contract is in status '{}'; only 'draft' contracts can be started",
+            current.status
+        )));
+    }
+
+    // If any sibling is already active, this one queues. Otherwise it
+    // claims the active slot directly.
+    let active_count: (i64,) = sqlx::query_as(
+        r#"SELECT COUNT(*)::BIGINT FROM directive_documents
+           WHERE directive_id = $1 AND status = 'active'"#,
+    )
+    .bind(current.directive_id)
+    .fetch_one(&mut *tx)
+    .await?;
+
+    let new_status = if active_count.0 > 0 { "queued" } else { "active" };
+
+    let updated = sqlx::query_as::<_, DirectiveDocument>(
+        r#"
+        UPDATE directive_documents
+           SET status = $2,
+               version = version + 1,
+               updated_at = NOW()
+         WHERE id = $1
+        RETURNING *
+        "#,
+    )
+    .bind(contract_id)
+    .bind(new_status)
+    .fetch_optional(&mut *tx)
+    .await?;
+
+    tx.commit().await?;
+    Ok(updated)
+}
+
+/// Pause an active contract — moves it back to `queued` so the next
+/// queued sibling can pick up the active slot. The orchestrator-daemon
+/// stop is the caller's responsibility.
+pub async fn pause_contract(
+    pool: &PgPool,
+    contract_id: Uuid,
+) -> Result<Option<DirectiveDocument>, RepositoryError> {
+    let mut tx = pool.begin().await?;
+
+    let current = sqlx::query_as::<_, DirectiveDocument>(
+        r#"SELECT * FROM directive_documents WHERE id = $1"#,
+    )
+    .bind(contract_id)
+    .fetch_optional(&mut *tx)
+    .await?;
+
+    let current = match current {
+        Some(c) => c,
+        None => return Ok(None),
+    };
+
+    if current.status != "active" {
+        return Err(RepositoryError::Validation(format!(
+            "contract is in status '{}'; only 'active' contracts can be paused",
+            current.status
+        )));
+    }
+
+    let updated = sqlx::query_as::<_, DirectiveDocument>(
+        r#"
+        UPDATE directive_documents
+           SET status = 'queued',
+               version = version + 1,
+               updated_at = NOW()
+         WHERE id = $1
+        RETURNING *
+        "#,
+    )
+    .bind(contract_id)
+    .fetch_optional(&mut *tx)
+    .await?;
+
+    // The slot is free — promote the next queued contract (lowest
+    // position, excluding the one we just paused).
+    promote_next_queued_contract(&mut tx, current.directive_id).await?;
+
+    tx.commit().await?;
+    Ok(updated)
+}
+
+/// Mark an active contract as `shipped` — the work is done. Optional
+/// pr_url / pr_branch are recorded if supplied. Promotes the next
+/// queued sibling to `active`.
+pub async fn complete_contract(
+    pool: &PgPool,
+    contract_id: Uuid,
+    pr_url: Option<&str>,
+    pr_branch: Option<&str>,
+) -> Result<Option<DirectiveDocument>, RepositoryError> {
+    let mut tx = pool.begin().await?;
+
+    let current = sqlx::query_as::<_, DirectiveDocument>(
+        r#"SELECT * FROM directive_documents WHERE id = $1"#,
+    )
+    .bind(contract_id)
+    .fetch_optional(&mut *tx)
+    .await?;
+
+    let current = match current {
+        Some(c) => c,
+        None => return Ok(None),
+    };
+
+    if current.status != "active" && current.status != "queued" {
+        return Err(RepositoryError::Validation(format!(
+            "contract is in status '{}'; only 'active' or 'queued' contracts can be completed",
+            current.status
+        )));
+    }
+
+    let updated = sqlx::query_as::<_, DirectiveDocument>(
+        r#"
+        UPDATE directive_documents
+           SET status = 'shipped',
+               pr_url = COALESCE($2, pr_url),
+               pr_branch = COALESCE($3, pr_branch),
+               shipped_at = NOW(),
+               version = version + 1,
+               updated_at = NOW()
+         WHERE id = $1
+        RETURNING *
+        "#,
+    )
+    .bind(contract_id)
+    .bind(pr_url)
+    .bind(pr_branch)
+    .fetch_optional(&mut *tx)
+    .await?;
+
+    promote_next_queued_contract(&mut tx, current.directive_id).await?;
+
+    tx.commit().await?;
+    Ok(updated)
+}
+
+/// Unlock a queued or active contract back to `draft` so the spec is
+/// editable again. If the contract was active, the slot frees and the
+/// next queued sibling auto-promotes.
+pub async fn unlock_contract(
+    pool: &PgPool,
+    contract_id: Uuid,
+) -> Result<Option<DirectiveDocument>, RepositoryError> {
+    let mut tx = pool.begin().await?;
+
+    let current = sqlx::query_as::<_, DirectiveDocument>(
+        r#"SELECT * FROM directive_documents WHERE id = $1"#,
+    )
+    .bind(contract_id)
+    .fetch_optional(&mut *tx)
+    .await?;
+
+    let current = match current {
+        Some(c) => c,
+        None => return Ok(None),
+    };
+
+    if current.status != "queued" && current.status != "active" {
+        return Err(RepositoryError::Validation(format!(
+            "contract is in status '{}'; only 'queued' or 'active' contracts can be unlocked",
+            current.status
+        )));
+    }
+
+    let was_active = current.status == "active";
+
+    let updated = sqlx::query_as::<_, DirectiveDocument>(
+        r#"
+        UPDATE directive_documents
+           SET status = 'draft',
+               version = version + 1,
+               updated_at = NOW()
+         WHERE id = $1
+        RETURNING *
+        "#,
+    )
+    .bind(contract_id)
+    .fetch_optional(&mut *tx)
+    .await?;
+
+    if was_active {
+        promote_next_queued_contract(&mut tx, current.directive_id).await?;
+    }
+
+    tx.commit().await?;
+    Ok(updated)
+}
+
+/// Find the lowest-position `queued` contract under a directive and
+/// flip it to `active`. No-op when no queued contract exists.
+///
+/// Caller must hold the parent transaction so the count → promote
+/// sequence stays atomic w.r.t. other lifecycle transitions.
+async fn promote_next_queued_contract(
+    tx: &mut sqlx::Transaction<'_, sqlx::Postgres>,
+    directive_id: Uuid,
+) -> Result<(), sqlx::Error> {
+    sqlx::query(
+        r#"
+        UPDATE directive_documents
+           SET status = 'active',
+               version = version + 1,
+               updated_at = NOW()
+         WHERE id = (
+             SELECT id FROM directive_documents
+              WHERE directive_id = $1 AND status = 'queued'
+              ORDER BY position ASC, created_at ASC
+              LIMIT 1
+         )
+        "#,
+    )
+    .bind(directive_id)
+    .execute(&mut **tx)
+    .await?;
+    Ok(())
 }
 
 /// Count the number of currently-active documents under a directive.
diff --git a/makima/src/server/handlers/directive_documents.rs b/makima/src/server/handlers/directive_documents.rs
index ed38ee4..23081b5 100644
--- a/makima/src/server/handlers/directive_documents.rs
+++ b/makima/src/server/handlers/directive_documents.rs
@@ -21,7 +21,7 @@ use utoipa::ToSchema;
 use uuid::Uuid;
 
 use crate::db::models::{DirectiveStep, Task};
-use crate::db::repository;
+use crate::db::repository::{self, RepositoryError};
 use crate::server::auth::Authenticated;
 use crate::server::messages::ApiError;
 use crate::server::state::SharedState;
@@ -60,6 +60,16 @@ pub struct ReorderDirectiveDocumentRequest {
     pub position: i32,
 }
 
+/// Body for `POST /api/v1/contracts/{document_id}/complete`. Both fields
+/// are optional — supplying them records PR provenance; leaving them off
+/// just marks the contract done.
+#[derive(Debug, Default, Deserialize, ToSchema)]
+#[serde(rename_all = "camelCase")]
+pub struct CompleteContractRequest {
+    pub pr_url: Option<String>,
+    pub pr_branch: Option<String>,
+}
+
 /// Body for `POST /api/v1/contracts/{document_id}/ship`.
 #[derive(Debug, Deserialize, ToSchema)]
 #[serde(rename_all = "camelCase")]
@@ -656,3 +666,213 @@ pub async fn reorder_contract(
         }
     }
 }
+
+// =============================================================================
+// Contract lifecycle: start / pause / complete / unlock
+//
+// State machine: `draft → queued → active → shipped → archived`. The
+// repository functions enforce transition validity and handle queue
+// auto-promotion when the active slot frees. Handlers here own auth
+// and mapping to HTTP status codes.
+// =============================================================================
+
+/// Common path: load + ownership-check, then dispatch a state-transition
+/// closure. Cuts the boilerplate from start/pause/complete/unlock down
+/// to a couple of lines each.
+async fn run_contract_transition<F, Fut>(
+    pool: sqlx::PgPool,
+    owner_id: Uuid,
+    contract_id: Uuid,
+    f: F,
+) -> impl IntoResponse
+where
+    F: FnOnce(sqlx::PgPool, Uuid) -> Fut,
+    Fut: std::future::Future<
+        Output = Result<Option<crate::db::models::DirectiveDocument>, RepositoryError>,
+    >,
+{
+    match load_owned_document(&pool, owner_id, contract_id).await {
+        Ok(Some(_)) => {}
+        Ok(None) => {
+            return (
+                StatusCode::NOT_FOUND,
+                Json(ApiError::new("NOT_FOUND", "Contract not found")),
+            )
+                .into_response();
+        }
+        Err(e) => {
+            return (
+                StatusCode::INTERNAL_SERVER_ERROR,
+                Json(ApiError::new("GET_FAILED", &e.to_string())),
+            )
+                .into_response();
+        }
+    }
+
+    match f(pool, contract_id).await {
+        Ok(Some(doc)) => Json(doc).into_response(),
+        Ok(None) => (
+            StatusCode::NOT_FOUND,
+            Json(ApiError::new("NOT_FOUND", "Contract not found")),
+        )
+            .into_response(),
+        Err(RepositoryError::Validation(msg)) => (
+            StatusCode::BAD_REQUEST,
+            Json(ApiError::new("VALIDATION", &msg)),
+        )
+            .into_response(),
+        Err(RepositoryError::VersionConflict { expected, actual }) => (
+            StatusCode::CONFLICT,
+            Json(ApiError::new(
+                "VERSION_CONFLICT",
+                &format!("expected version {}, actual {}", expected, actual),
+            )),
+        )
+            .into_response(),
+        Err(RepositoryError::Database(e)) => {
+            tracing::error!("Contract transition failed: {}", e);
+            (
+                StatusCode::INTERNAL_SERVER_ERROR,
+                Json(ApiError::new("TRANSITION_FAILED", &e.to_string())),
+            )
+                .into_response()
+        }
+    }
+}
+
+/// Lock & start a draft contract. If a sibling is already `active`,
+/// this contract goes to `queued`; otherwise it activates immediately.
+#[utoipa::path(
+    post,
+    path = "/api/v1/contracts/{document_id}/start",
+    params(("document_id" = Uuid, Path, description = "Contract ID")),
+    responses(
+        (status = 200, description = "Contract started", body = crate::db::models::DirectiveDocument),
+        (status = 400, description = "Invalid state transition", body = ApiError),
+        (status = 404, description = "Not found", body = ApiError),
+    ),
+    security(("bearer_auth" = []), ("api_key" = [])),
+    tag = "Directive Documents"
+)]
+pub async fn start_contract(
+    State(state): State<SharedState>,
+    Authenticated(auth): Authenticated,
+    Path(document_id): Path<Uuid>,
+) -> impl IntoResponse {
+    let Some(pool) = state.db_pool.clone() else {
+        return (
+            StatusCode::SERVICE_UNAVAILABLE,
+            Json(ApiError::new("DB_UNAVAILABLE", "Database not configured")),
+        )
+            .into_response();
+    };
+    run_contract_transition(pool, auth.owner_id, document_id, |pool, id| async move {
+        repository::start_contract(&pool, id).await
+    })
+    .await
+    .into_response()
+}
+
+/// Pause an active contract — moves it back to `queued` and lets the
+/// next queued sibling take the active slot. The orchestrator daemon
+/// stop is the caller's responsibility.
+#[utoipa::path(
+    post,
+    path = "/api/v1/contracts/{document_id}/pause",
+    params(("document_id" = Uuid, Path, description = "Contract ID")),
+    responses(
+        (status = 200, description = "Contract paused", body = crate::db::models::DirectiveDocument),
+        (status = 400, description = "Invalid state transition", body = ApiError),
+        (status = 404, description = "Not found", body = ApiError),
+    ),
+    security(("bearer_auth" = []), ("api_key" = [])),
+    tag = "Directive Documents"
+)]
+pub async fn pause_contract(
+    State(state): State<SharedState>,
+    Authenticated(auth): Authenticated,
+    Path(document_id): Path<Uuid>,
+) -> impl IntoResponse {
+    let Some(pool) = state.db_pool.clone() else {
+        return (
+            StatusCode::SERVICE_UNAVAILABLE,
+            Json(ApiError::new("DB_UNAVAILABLE", "Database not configured")),
+        )
+            .into_response();
+    };
+    run_contract_transition(pool, auth.owner_id, document_id, |pool, id| async move {
+        repository::pause_contract(&pool, id).await
+    })
+    .await
+    .into_response()
+}
+
+/// Mark an active contract as `shipped` (work done). PR url + branch
+/// are optional — pass them to record provenance, leave them off to
+/// just close out the contract. Auto-promotes the next queued sibling.
+#[utoipa::path(
+    post,
+    path = "/api/v1/contracts/{document_id}/complete",
+    params(("document_id" = Uuid, Path, description = "Contract ID")),
+    request_body = CompleteContractRequest,
+    responses(
+        (status = 200, description = "Contract completed", body = crate::db::models::DirectiveDocument),
+        (status = 400, description = "Invalid state transition", body = ApiError),
+        (status = 404, description = "Not found", body = ApiError),
+    ),
+    security(("bearer_auth" = []), ("api_key" = [])),
+    tag = "Directive Documents"
+)]
+pub async fn complete_contract(
+    State(state): State<SharedState>,
+    Authenticated(auth): Authenticated,
+    Path(document_id): Path<Uuid>,
+    Json(req): Json<CompleteContractRequest>,
+) -> impl IntoResponse {
+    let Some(pool) = state.db_pool.clone() else {
+        return (
+            StatusCode::SERVICE_UNAVAILABLE,
+            Json(ApiError::new("DB_UNAVAILABLE", "Database not configured")),
+        )
+            .into_response();
+    };
+    run_contract_transition(pool, auth.owner_id, document_id, move |pool, id| async move {
+        repository::complete_contract(&pool, id, req.pr_url.as_deref(), req.pr_branch.as_deref()).await
+    })
+    .await
+    .into_response()
+}
+
+/// Unlock a queued or active contract back to `draft` so its spec is
+/// editable again. If the contract was active, the slot frees and the
+/// next queued sibling auto-promotes.
+#[utoipa::path(
+    post,
+    path = "/api/v1/contracts/{document_id}/unlock",
+    params(("document_id" = Uuid, Path, description = "Contract ID")),
+    responses(
+        (status = 200, description = "Contract unlocked", body = crate::db::models::DirectiveDocument),
+        (status = 400, description = "Invalid state transition", body = ApiError),
+        (status = 404, description = "Not found", body = ApiError),
+    ),
+    security(("bearer_auth" = []), ("api_key" = [])),
+    tag = "Directive Documents"
+)]
+pub async fn unlock_contract(
+    State(state): State<SharedState>,
+    Authenticated(auth): Authenticated,
+    Path(document_id): Path<Uuid>,
+) -> impl IntoResponse {
+    let Some(pool) = state.db_pool.clone() else {
+        return (
+            StatusCode::SERVICE_UNAVAILABLE,
+            Json(ApiError::new("DB_UNAVAILABLE", "Database not configured")),
+        )
+            .into_response();
+    };
+    run_contract_transition(pool, auth.owner_id, document_id, |pool, id| async move {
+        repository::unlock_contract(&pool, id).await
+    })
+    .await
+    .into_response()
+}
diff --git a/makima/src/server/handlers/files.rs b/makima/src/server/handlers/files.rs
index 05e871c..711be41 100644
--- a/makima/src/server/handlers/files.rs
+++ b/makima/src/server/handlers/files.rs
@@ -277,6 +277,11 @@ pub async fn update_file(
             )
                 .into_response()
         }
+        Err(RepositoryError::Validation(msg)) => (
+            StatusCode::BAD_REQUEST,
+            Json(ApiError::new("VALIDATION", &msg)),
+        )
+            .into_response(),
     }
 }
 
diff --git a/makima/src/server/handlers/mesh.rs b/makima/src/server/handlers/mesh.rs
index 63b1827..be5387e 100644
--- a/makima/src/server/handlers/mesh.rs
+++ b/makima/src/server/handlers/mesh.rs
@@ -467,6 +467,11 @@ pub async fn update_task(
             )
                 .into_response()
         }
+        Err(RepositoryError::Validation(msg)) => (
+            StatusCode::BAD_REQUEST,
+            Json(ApiError::new("VALIDATION", &msg)),
+        )
+            .into_response(),
     }
 }
 
diff --git a/makima/src/server/handlers/versions.rs b/makima/src/server/handlers/versions.rs
index 15118d6..bb1b00c 100644
--- a/makima/src/server/handlers/versions.rs
+++ b/makima/src/server/handlers/versions.rs
@@ -203,5 +203,10 @@ pub async fn restore_version(
             )
                 .into_response()
         }
+        Err(RepositoryError::Validation(msg)) => (
+            StatusCode::BAD_REQUEST,
+            Json(ApiError::new("VALIDATION", &msg)),
+        )
+            .into_response(),
     }
 }
diff --git a/makima/src/server/mod.rs b/makima/src/server/mod.rs
index 68d3dea..a3a1886 100644
--- a/makima/src/server/mod.rs
+++ b/makima/src/server/mod.rs
@@ -242,6 +242,22 @@ pub fn make_router(state: SharedState) -> Router {
             post(directive_documents::reorder_contract),
         )
         .route(
+            "/contracts/{document_id}/start",
+            post(directive_documents::start_contract),
+        )
+        .route(
+            "/contracts/{document_id}/pause",
+            post(directive_documents::pause_contract),
+        )
+        .route(
+            "/contracts/{document_id}/complete",
+            post(directive_documents::complete_contract),
+        )
+        .route(
+            "/contracts/{document_id}/unlock",
+            post(directive_documents::unlock_contract),
+        )
+        .route(
             "/contracts/{document_id}/tasks",
             get(directive_documents::list_document_tasks),
         )
diff --git a/makima/src/server/openapi.rs b/makima/src/server/openapi.rs
index 7ddaf1b..184d12a 100644
--- a/makima/src/server/openapi.rs
+++ b/makima/src/server/openapi.rs
@@ -124,6 +124,10 @@ use crate::server::messages::{ApiError, AudioEncoding, StartMessage, StopMessage
         directive_documents::ship_document,
         directive_documents::archive_document,
         directive_documents::reorder_contract,
+        directive_documents::start_contract,
+        directive_documents::pause_contract,
+        directive_documents::complete_contract,
+        directive_documents::unlock_contract,
         directive_documents::list_document_tasks,
         // Order endpoints
         orders::list_orders,
@@ -233,6 +237,7 @@ use crate::server::messages::{ApiError, AudioEncoding, StartMessage, StopMessage
             directive_documents::UpdateDirectiveDocumentRequest,
             directive_documents::ShipDirectiveDocumentRequest,
             directive_documents::ReorderDirectiveDocumentRequest,
+            directive_documents::CompleteContractRequest,
             directive_documents::DocumentTasksResponse,
             // Order schemas
             Order,