fix(doctor): guard nullable engine in supervisor DB-lock fallback (#2227)

Follow-up to the supervisor-visibility commit: doctor's engine binding is
BrainEngine | null, so the inspectLock fallback must guard on a non-null engine
(tsc TS2345). No behavior change — a null engine simply skips the DB-lock probe
and falls back to the pidfile reading, as before.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/commands/doctor.ts

@@ -4401,7 +4401,7 @@ export async function buildChecks(
     // Consult the queue-scoped DB singleton lock (#1849, HOME-independent) before
     // warning. PID-reuse-safe (isLockHolderLive keys on lock freshness).
     let detectedViaDbLock = false;
-    if (!pidfileRunning) {
+    if (!pidfileRunning && engine) {
       try {
         const { inspectLock, isLockHolderLive } = await import('../core/db-lock.ts');
         const { supervisorLockId, SUPERVISOR_LOCK_TTL_MIN } = await import('../core/minions/supervisor.ts');