fix: self-healing stale worktree recovery in SimpleGitWorktreeManager

When git worktree add fails with "branch already used by worktree at
<path>", parse the stale path, force-remove it, and retry once. Fixes
blocked task retries where the old agent-workdirs directory still exists
on disk and git worktree prune alone can't clear the reference.

apps/server/git/manager.ts

@@ -85,10 +85,32 @@ export class SimpleGitWorktreeManager implements WorktreeManager {
       }
       // Prune stale worktree references before adding new one
       await this.git.raw(['worktree', 'prune']);
-      await this.git.raw(['worktree', 'add', worktreePath, branch]);
+      try {
+        await this.git.raw(['worktree', 'add', worktreePath, branch]);
+      } catch (err) {
+        const stalePath = this.parseStaleWorktreePath(err);
+        if (stalePath) {
+          log.warn({ branch, stalePath }, 'branch locked to stale worktree, force-removing and retrying');
+          await this.forceRemoveWorktree(stalePath);
+          await this.git.raw(['worktree', 'add', worktreePath, branch]);
+        } else {
+          throw err;
+        }
+      }
     } else {
       // git worktree add -b <branch> <path> <base-branch>
-      await this.git.raw(['worktree', 'add', '-b', branch, worktreePath, baseBranch]);
+      try {
+        await this.git.raw(['worktree', 'add', '-b', branch, worktreePath, baseBranch]);
+      } catch (err) {
+        const stalePath = this.parseStaleWorktreePath(err);
+        if (stalePath) {
+          log.warn({ branch, stalePath }, 'branch locked to stale worktree, force-removing and retrying');
+          await this.forceRemoveWorktree(stalePath);
+          await this.git.raw(['worktree', 'add', '-b', branch, worktreePath, baseBranch]);
+        } else {
+          throw err;
+        }
+      }
     }
 
     const worktree: Worktree = {
@@ -300,6 +322,25 @@ export class SimpleGitWorktreeManager implements WorktreeManager {
     }
   }
 
+  /** Parse stale worktree path from "already used by worktree at '<path>'" git error. */
+  private parseStaleWorktreePath(err: unknown): string | null {
+    const msg = err instanceof Error ? err.message : String(err);
+    const match = msg.match(/already used by worktree at '([^']+)'/);
+    return match ? match[1] : null;
+  }
+
+  /** Force-remove a stale worktree (git ref + directory). */
+  private async forceRemoveWorktree(worktreePath: string): Promise<void> {
+    try {
+      await this.git.raw(['worktree', 'remove', '--force', worktreePath]);
+    } catch {
+      // Corrupted state fallback: delete directory and prune
+      const { rm } = await import('node:fs/promises');
+      await rm(worktreePath, { recursive: true, force: true });
+      await this.git.raw(['worktree', 'prune']);
+    }
+  }
+
   /**
    * Parse the porcelain output of git worktree list.
    */