fix: storeThought orphaned lock → defer unlock

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

@@ -1895,6 +1895,7 @@ func (t *Thinker) buildOpenAITools() []llm.OpenAITool {
 // storeThought 存储思考结果到待推送队列，并异步持久化到 memory-service
 func (t *Thinker) storeThought(content string, toolCallsJSON string, toolCallCount int) {
 	t.muLock()
+	defer t.muUnlock()
 	t.pendingThoughts = append(t.pendingThoughts, &PendingThought{
 		Content:   content,
 		CreatedAt: time.Now(),
@@ -1951,7 +1952,6 @@ func (t *Thinker) storeThought(content string, toolCallsJSON string, toolCallCou
 		copy := *proactiveTarget
 		targetCopy = &copy
 	}
-	t.muUnlock()
 
 	log.Printf("[后台思考] 思考已存储 (当前累积 %d 条待推送思考)", len(t.pendingThoughts))