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>

1 files changed, 39 insertions, 4 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!(