fix: prevent directive step failure when PR branch is deleted after merge

Stop using pr_branch as base branch for step tasks since it may be
auto-deleted by GitHub after PR merge. Instead always use
continue_from_task_id or fall back to base_branch. Also add a safety
net in create_worktree that detects when a base branch ref no longer
exists and falls back to the repo's default branch.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

2 files changed, 56 insertions, 32 deletions

diff --git a/makima/src/daemon/worktree/manager.rs b/makima/src/daemon/worktree/manager.rs
index c27bcf6..30618ea 100644
--- a/makima/src/daemon/worktree/manager.rs
+++ b/makima/src/daemon/worktree/manager.rs
@@ -484,7 +484,9 @@ impl WorktreeManager {
                 "Creating worktree with new branch"
             );
 
-            // Prefer origin/{base_branch} to get latest remote state
+            // Prefer origin/{base_branch} to get latest remote state.
+            // If neither origin/{base_branch} nor {base_branch} exist (e.g. PR branch
+            // was deleted after merge), fall back to the repo's default branch.
             let origin_ref = format!("origin/{}", base_branch);
             let has_origin_ref = Command::new("git")
                 .args(["rev-parse", "--verify", &format!("refs/remotes/{}", origin_ref)])
@@ -494,10 +496,43 @@ impl WorktreeManager {
                 .map(|o| o.status.success())
                 .unwrap_or(false);
 
-            let start_point = if has_origin_ref {
-                origin_ref.as_str()
+            let has_local_ref = if !has_origin_ref {
+                Command::new("git")
+                    .args(["rev-parse", "--verify", &format!("refs/heads/{}", base_branch)])
+                    .current_dir(source_repo)
+                    .output()
+                    .await
+                    .map(|o| o.status.success())
+                    .unwrap_or(false)
             } else {
-                base_branch
+                false // don't need to check — we already have origin ref
+            };
+
+            let start_point: String = if has_origin_ref {
+                origin_ref
+            } else if has_local_ref {
+                base_branch.to_string()
+            } else {
+                // Branch doesn't exist (likely deleted after PR merge) — use default branch
+                tracing::warn!(
+                    task_id = %task_id,
+                    base_branch = %base_branch,
+                    "Base branch ref not found, falling back to default branch"
+                );
+                let default_branch = self.detect_default_branch(source_repo).await?;
+                let default_origin = format!("origin/{}", default_branch);
+                let has_default_origin = Command::new("git")
+                    .args(["rev-parse", "--verify", &format!("refs/remotes/{}", default_origin)])
+                    .current_dir(source_repo)
+                    .output()
+                    .await
+                    .map(|o| o.status.success())
+                    .unwrap_or(false);
+                if has_default_origin {
+                    default_origin
+                } else {
+                    default_branch
+                }
             };
 
             tracing::info!(
diff --git a/makima/src/orchestration/directive.rs b/makima/src/orchestration/directive.rs
index a6bb85b..af6b18c 100644
--- a/makima/src/orchestration/directive.rs
+++ b/makima/src/orchestration/directive.rs
@@ -103,36 +103,25 @@ impl DirectiveOrchestrator {
             };
             let mut continue_from_task_id = dep_tasks.first().map(|d| d.task_id);
 
-            // If no dependency tasks resolved, try to continue from previous work:
-            // 1) Use the directive's PR branch as base (contains all previous merged work)
-            // 2) Fall back to the last completed step's task for worktree continuation
+            // If no dependency tasks resolved, try to continue from the last completed step's worktree.
+            // We never use pr_branch as base because it may have been deleted after PR merge.
             let effective_base_branch = if continue_from_task_id.is_none() {
-                if step.pr_branch.is_some() {
-                    tracing::info!(
-                        step_id = %step.step_id,
-                        pr_branch = ?step.pr_branch,
-                        "Step has no deps — using directive PR branch as base"
-                    );
-                    step.pr_branch.as_deref()
-                } else {
-                    // No PR branch yet — try to continue from the last completed step's worktree
-                    match repository::get_last_completed_step_task_id(
-                        &self.pool,
-                        step.directive_id,
-                    )
-                    .await
-                    {
-                        Ok(Some(task_id)) => {
-                            tracing::info!(
-                                step_id = %step.step_id,
-                                continue_from = %task_id,
-                                "Step has no deps, no PR branch — continuing from last completed task"
-                            );
-                            continue_from_task_id = Some(task_id);
-                            step.base_branch.as_deref()
-                        }
-                        _ => step.base_branch.as_deref(),
+                match repository::get_last_completed_step_task_id(
+                    &self.pool,
+                    step.directive_id,
+                )
+                .await
+                {
+                    Ok(Some(task_id)) => {
+                        tracing::info!(
+                            step_id = %step.step_id,
+                            continue_from = %task_id,
+                            "Step has no deps — continuing from last completed task"
+                        );
+                        continue_from_task_id = Some(task_id);
+                        step.base_branch.as_deref()
                     }
+                    _ => step.base_branch.as_deref(),
                 }
             } else {
                 step.base_branch.as_deref()