fix(FN-4811): recover from validation-failed remove + collapse broken FN-4806 nested branches

Two follow-ups stacked on the FN-4811 active-worktree liveness gate:

1. Stale conflict-path recovery (FN-4813 production failure)

   When 'git worktree remove --force' fails with 'fatal: validation failed,
   cannot remove working tree', the worktree directory is missing on disk
   and the git admin entry is stale. Without this recovery, every retry of
   tryCreateWorktree on a stale conflict path failed 3 times with
   'automatic cleanup failed', leaving tasks unable to create worktrees.

   cleanupConflictingWorktree now catches that specific error class, runs
   'git worktree prune' to drop the stale admin entry, best-effort deletes
   the branch, and returns success so the caller can proceed.

   Implementation note: the original attempt used existsSync(worktreePath)
   as a pre-check, but vitest's vi.clearAllMocks() can leave the existsSync
   mock returning undefined, causing the new branch to fire inside tests
   that didn't expect it and leading to worker OOM in
   executor-worktree.test.ts. The error-class-based catch is robust against
   mock state and matches the real production failure signal exactly.

2. Collapsed broken FN-4806 nested branches

   The previous FN-4806 refactor (commit 087b1a766) accidentally nested the
   genuine 'agent finished without calling fn_task_done after N retries'
   failure path INSIDE the silent-recovery branch, meaning ordinary
   failures were being silently requeued (no status=failed, no onError, no
   retry-budget burn) instead of being surfaced.

   Restored the clean two-branch structure:

     } else if (retryAbortedDueToReclaim) {
       // silent recovery (FN-4806)
     } else {
       // genuine no-fn_task_done exhaustion: mark failed, onError, burn budget
     }

   Also clears baseCommitSha on silent recovery (matches the parallel
   session-start-failure path's metadata clearing).

Tests:

  - Adds 'FN-4811 follow-up (FN-4813): recovers from validation failed'
    case to active-worktree-removal-liveness.test.ts (12 total cases).
  - executor-recovery.test.ts no-fn_task_done reclaim coverage now
    asserts baseCommitSha is cleared.
  - executor-recovery.test.ts 'does not mark task as failed when invalid
    transition error occurs on completion' regression fixed by restoring
    the failure-path branch.
  - executor-core.test.ts 'still enforces fn_task_done requirement in
    fast mode' restored.

Full engine suite: 307 files, 5037 tests pass, 1 skipped. Lint clean.

Fusion-Task-Id: FN-4811

.changeset/FN-4811-stale-conflict-path-recovery.md

@@ -0,0 +1,11 @@
+---
+"@runfusion/fusion": patch
+---
+
+fix(FN-4811): recover from "validation failed, cannot remove working tree" + collapse broken FN-4806 nested branches
+
+Two follow-ups stacked on the FN-4811 active-worktree liveness gate:
+
+1. **Stale conflict-path recovery (was breaking real tasks).** When `git worktree remove --force` fails with `fatal: validation failed, cannot remove working tree`, the worktree directory is missing on disk and the git admin entry is stale. `cleanupConflictingWorktree` now catches that specific error, runs `git worktree prune`, best-effort deletes the branch, and returns success — so the caller can proceed with worktree creation instead of failing 3× with "automatic cleanup failed" (FN-4813 production failure).
+
+2. **Collapsed broken FN-4806 nested branches.** The previous FN-4806 refactor accidentally nested the genuine "agent finished without calling fn_task_done" failure path inside the silent-recovery branch, so ordinary failures were being silently requeued instead of marked failed. Restored the clean two-branch structure: `else if (retryAbortedDueToReclaim)` silent-recovers, `else` marks failed/onError/burns budget. Also clears `baseCommitSha` on silent recovery (matches the parallel session-start-failure path).