feat(FN-5627): self-heal transient merge failures stuck at mergeRetries=3

After the FN-5627 merger fix (b2d547eae, 230f6f45b) landed, two in-review
tasks (FN-5628, FN-5632) remained stuck at mergeRetries=3 with
status=failed because the merger correctly identified transient failure
classes but had no auto-recovery path \u2014 the AUTO_MERGE_COOLDOWN_MS reset
takes hours and gives up too easily.

Failure classes covered:
- lease-handoff-failed: target-not-queued (FN-5353/FN-5363 race where the
  merge queue lease was cleared between enqueue and handoff acquisition).
- Legacy same-SHA spurious 'Integration branch X advanced concurrently
  (expected SHA, observed SHA)' errors from pre-FN-5627 code paths.

Implementation:
- New MergeDetails.transientRecoveryCount field tracks per-task recovery
  attempts, bounded by MAX_TRANSIENT_MERGE_RECOVERIES = 2.
- New classifyTransientMergeError() string matcher in self-healing.ts
  identifies recoverable classes by error pattern. Returns null for
  genuine merge failures (verification, conflicts, real concurrent
  advances with different SHAs).
- SelfHealingManager.recoverTransientMergeFailures() sweep finds
  matching in-review tasks, resets mergeRetries=0, clears status/error,
  increments recovery count, re-enqueues via requeueForAutoMerge.
- Wired into BOTH startup recovery and periodic Batch 2 maintenance loop.
- Emits merger:transient-failure-auto-recovered (recovered) and
  merger:transient-failure-budget-exhausted (terminal) audit events.

No-op when autoMerge=false, requeueForAutoMerge not wired, or pause
active. Repeat-suppression on budget-exhausted emit via error marker
[transient-recovery-budget-exhausted] to prevent log spam.

Tests (6 new):
- target-not-queued recovery path
- spurious-concurrent-advance-same-sha recovery path (legacy)
- genuine concurrent-advance (different SHAs) NOT recovered
- non-transient failures NOT recovered (verification, conflicts)
- budget exhaustion emits marker once, no further requeue
- autoMerge=false no-op

Engine suite: 6157 tests pass (6 new).

In-flight: FN-5628 and FN-5632 were manually reset via SQL so the
already-running engine (which has the FN-5627 merger fix) can re-attempt
their merges before this self-healing path lands and reloads. Future
occurrences self-recover.

Fusion-Task-Id: FN-5627

packages/core/src/types.ts

@@ -1389,6 +1389,16 @@ export interface MergeDetails {
     warnedAt: string;
     reason: string;
   };
+  /**
+   * FN-5627 follow-up: counts how many times self-healing
+   * `recoverTransientMergeFailures` has reset this task's `mergeRetries` and
+   * re-enqueued it after a transient merge failure (e.g., `target-not-queued`
+   * lease handoff race, or a misclassified same-SHA spurious concurrent-advance
+   * left over from pre-FN-5627 code paths). Bounded by `MAX_TRANSIENT_MERGE_RECOVERIES`
+   * (2) to avoid infinite recovery loops on genuinely-stuck tasks. Distinct from
+   * `task.mergeRetries`, which counts in-cycle aiMergeTask retries.
+   */
+  transientRecoveryCount?: number;
 }
 
 /** Represents an agent's checkout lease on a task. */