fix(FN-5627): suppress ntfy notifications for transient merge failures the engine auto-recovers

Even with FN-5627's merger TOCTOU fix + transient-failure self-healing
sweep + safety-fallback auto-prerebase landed, the merger can still hit
transient failure classes (lease handoff races, brief same-SHA non-FF
advances) for tasks whose branches are particularly out-of-sync. The
self-healing sweep auto-recovers them within bounded budget \u2014 but each
individual failure cycle was firing a ntfy alarm before the recovery
cleared the failed state, producing user-facing alarm spam for tasks
that were never actually stuck.

Two layers of fix:

1. NotificationService.handleTaskUpdated now classifies task.error via
   the new shared classifyTransientMergeError helper before scheduling
   the deferred failure notification. Transient classes
   (lease-handoff-target-not-queued, spurious-concurrent-advance-same-sha)
   get logged as suppressed and never schedule a ntfy timer.

2. Defense-in-depth: fireDeferredFailureNotification re-classifies the
   error at dispatch time, so a failure scheduled before the suppression
   landed on a newer cycle still suppresses if the error matches a
   transient class.

The classifier itself moved from self-healing.ts to a new logger-free
transient-merge-error-classifier.ts module so consumers in
NotificationService don't pull createLogger through the import chain and
break test mocks of ../logger.js (per project-memory rule about new
modules using createLogger). self-healing.ts re-exports the symbol for
backward compatibility.

Log prefix for the recovery actions also changed from
'[FN-5627] Auto-recovering...' to 'Auto-recovered:' so that
NotificationService.maybeSuppressTransientFailedNotification's existing
/^Auto-recovered:/ log-prefix check cancels any already-scheduled failure
notification when the sweep runs mid-grace-window.

Tests (3 new):
- transient lease-handoff-target-not-queued failure NOT notified
- transient spurious-concurrent-advance-same-sha failure NOT notified
- genuine different-SHAs concurrent-advance still notifies (control)

Engine suite: 6166 tests pass.

Fusion-Task-Id: FN-5627

packages/engine/src/project-engine.ts

@@ -1544,9 +1544,12 @@ export class ProjectEngine {
                 runtimeLog.warn(
                   `Auto-merge: ${taskId} fast-path REFUSED — auto-recovering (attempt ${nextRetries}/${ProjectEngine.MAX_AUTO_MERGE_RETRIES}): ${reachability.reason}: ${reachability.diagnostic}`,
                 );
+                // Prefix MUST be "Auto-recovered:" so NotificationService's
+                // maybeSuppressTransientFailedNotification cancels the pending
+                // ntfy fired off the underlying task:failed event.
                 await store.logEntry(
                   taskId,
-                  `[FN-5627] Auto-merge fast-path refused — cleared poisoned mergeDetails (commit ${shortSha} not reachable from ${integrationBranchForGate}, ${reachability.reason}). Re-enqueueing for fresh merge attempt ${nextRetries}/${ProjectEngine.MAX_AUTO_MERGE_RETRIES}.`,
+                  `Auto-recovered: fast-path refused — cleared poisoned mergeDetails (commit ${shortSha} not reachable from ${integrationBranchForGate}, ${reachability.reason}). Re-enqueueing for fresh merge attempt ${nextRetries}/${ProjectEngine.MAX_AUTO_MERGE_RETRIES} [FN-5627].`,
                 );
                 await store.updateTask(taskId, {
                   mergeDetails: cleanedMergeDetails,