fix: 移除有问题的runtime.SetFinalizer

backend/ai-core/internal/background/thinker.go

@@ -2661,6 +2661,10 @@ func (t *Thinker) muLock() {
 
 // muUnlock releases t.mu and clears the tracking.
 func (t *Thinker) muUnlock() {
+	held := time.Since(t.muLockedAt)
+	if held > 5*time.Second {
+		log.Printf("[debug] muUnlock: lock held for %v by %s", held.Round(time.Millisecond), t.muLockedBy)
+	}
 	t.muLockedBy = ""
 	t.mu.Unlock()
 }