feat: 提醒到期 → LLM生成自然提醒语 → 推送

- Gateway ReminderScheduler 到期时调用 ai-core /api/v1/internal/reminder-trigger
- ai-core 用 LLM 生成温柔俏皮的提醒语，通过 messagePusher 推送
- Thinker 新增 TriggerReminderMessage 方法
- 环境变量: AI_CORE_URL (Gateway 已有)

Co-Authored-By: Claude <noreply@anthropic.com>

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

@@ -350,6 +350,26 @@ func (t *Thinker) UpdatePresence(online bool, sessionID string) {
 	}
 }
 
+// TriggerReminderMessage pushes a reminder message generated by LLM to the user.
+// Called by the internal reminder-trigger endpoint when a reminder fires.
+func (t *Thinker) TriggerReminderMessage(userID, sessionID, message string) {
+	t.mu.Lock()
+	pusher := t.messagePusher
+	pushSessionID := sessionID
+	if pushSessionID == "" {
+		pushSessionID = t.activeSessionID
+	}
+	if pushSessionID == "" {
+		pushSessionID = t.adminSessionID
+	}
+	t.mu.Unlock()
+
+	if pusher != nil && message != "" {
+		log.Printf("[提醒推送] 推送LLM提醒: user=%s session=%s msg=%s", userID, pushSessionID, message)
+		pusher(userID, pushSessionID, message)
+	}
+}
+
 // ThinkerConfig 后台思考配置
 type ThinkerConfig struct {
 	Enabled          bool