fix: buildThinkingUserPrompt嵌套锁死锁 + 管理员群聊路由优化

- thinker.go:1662/1762 defer t.muUnlock→立即解锁，消除buildThinkingUserPrompt自死锁
  (1662持锁后1762再Lock→同goroutine自死锁→23h锁泄露→3769 goroutine泄漏)
- platform-bridge: 管理员在群里@别人走group_ambient而非normal，避免频繁插嘴

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

@@ -1660,8 +1660,8 @@ func (t *Thinker) buildThinkingUserPrompt(
 		sb.WriteString("刚有人和你聊完天。你想自然地在心里回味一下刚才的对话……\n")
 	case "silence":
 		t.muLock()
-		defer t.muUnlock()
 		silenceDuration := time.Since(t.lastUserTime())
+		t.muUnlock()
 
 		sb.WriteString(fmt.Sprintf("已经大约 %s 没有说话了。你有点想知道大家在做什么……\n",
 			formatDurationHuman(silenceDuration)))
@@ -1760,7 +1760,7 @@ func (t *Thinker) buildThinkingUserPrompt(
 
 		// OBv11 platform identity and available channels for proactive messaging.
 		t.muLock()
-		defer t.muUnlock()
+		t.muUnlock()
 		qqChannels := t.platformChannels
 		botUIDs := t.botUIDs
 		activeSID := t.activeSessionID

backend/platform-bridge/cmd/main.go

@@ -211,8 +211,16 @@ func main() {
 			response = &bridge.UnifiedResponse{Messages: []bridge.ResponseMessage{{DisplayType: "silent"}}, Platform: msg.Platform}
 
 		case isAdmin:
-			msg.RouteType = "normal"
+			// 管理员在群里@别人(非bot)时，用 group_ambient 避免昔涟频繁插嘴
-			response, routeErr = forwardToAICore(cfg, msg, "text", chatUserID, groupSessionID, imageURLs, videoURLs, voiceURLs, isAdmin)
+			if msg.ChannelType == "group" && !isBotMentioned {
+				msg.RouteType = "group_ambient"
+				namespace := buildMemoryNamespace(msg.Platform, msg.ChannelType, msg.ChannelID)
+				fireSilent(namespace, imageURLs, videoURLs, voiceURLs)
+				response, routeErr = forwardToAICore(cfg, msg, "group_ambient", chatUserID, groupSessionID, imageURLs, videoURLs, voiceURLs, isAdmin)
+			} else {
+				msg.RouteType = "normal"
+				response, routeErr = forwardToAICore(cfg, msg, "text", chatUserID, groupSessionID, imageURLs, videoURLs, voiceURLs, isAdmin)
+			}
 
 		case isBotMentioned:
 			msg.RouteType = "normal"