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