feat(gateway): 静默定时任务增加主动送达提示，避免结果静默丢失

silent_agent_task 的最终响应发往 scheduler/ 虚拟会话、被出站分发器跳过，成功时用户收不到任何消息，只能依赖 agent 主动调用 send_session_message。

- 定时任务系统提示词仅对 scheduler/ 虚拟会话追加送达提示（避免普通 agent_task 重复发送）

- scheduler_manage 工具描述新增 Silent Mode Delivery 指引：用户期望收到结果时，prompt 必须要求用 send_session_message 交付

src/gateway/execution.rs

@@ -74,6 +74,11 @@ impl CompactionSink for CompactionSinkImpl {
 
 const SCHEDULED_TASK_EXECUTION_SYSTEM_PROMPT: &str = "系统说明：当前输入来自一次已经触发的定时任务执行。你现在需要执行任务内容本身，而不是创建、修改、恢复、暂停或查询新的定时任务。除非当前任务内容明确要求管理调度器，否则不要调用任何定时任务管理工具；像“每小时”、“每天”、“cron”、“定时”等词，只应视为任务背景，不应再解释为新的建任务请求。";
 
+/// 静默（后台）定时任务的送达提示：最终响应发往虚拟会话，不会自动推送给用户，
+/// 必须显式调用 send_session_message 才能把结果送达用户会话。
+/// 仅在 scheduler/ 虚拟会话（silent_agent_task）中追加，避免普通 agent_task 重复发送。
+const SCHEDULED_TASK_SILENT_DELIVERY_HINT: &str = "特别注意：本次定时任务运行在后台独立会话中，你的最终响应不会自动发送给用户。如果任务需要向用户交付结果或发出通知，必须主动调用 send_session_message 工具把内容发送到当前会话，否则用户不会收到任何消息。";
+
 pub(crate) fn compose_scheduled_task_system_prompt(system_prompt: Option<&str>) -> String {
     match system_prompt
         .map(str::trim)
@@ -497,8 +502,16 @@ impl AgentExecutionService {
             session_guard.ensure_chat_loaded(request.chat_id, original_topic_id.as_deref())?;
             session_guard.ensure_agent_prompt_before_user_message(request.chat_id)?;
 
-            let scheduled_system_prompt =
+            let mut scheduled_system_prompt =
                 compose_scheduled_task_system_prompt(request.system_prompt);
+            // 静默任务（scheduler/ 虚拟会话）的最终响应不会送达用户，
+            // 提示 agent 必须用 send_session_message 主动交付结果
+            if is_scheduler_chat_id(request.chat_id) {
+                scheduled_system_prompt = format!(
+                    "{}\n{}",
+                    scheduled_system_prompt, SCHEDULED_TASK_SILENT_DELIVERY_HINT
+                );
+            }
             session_guard.append_persisted_message(
                 request.chat_id,
                 original_topic_id.as_deref(),

src/tools/scheduler_manage.rs

@@ -41,6 +41,9 @@ impl Tool for SchedulerManageTool {
         IMPORTANT - Default to Silent Mode: \
         When users request scheduled tasks without explicitly specifying the mode, default to silent_agent_task instead of agent_task. Silent mode is preferred for most automated tasks because it runs in a dedicated background session without cluttering the main conversation. Only use agent_task when the user explicitly wants interactive execution in the main chat. \
         \
+        IMPORTANT - Silent Mode Delivery: \
+        A silent_agent_task runs in a background session and its final response is NOT automatically sent to the user's conversation; only execution failures trigger a notification. Therefore, when the user expects to receive the task's result (a report, a status, an alert, etc.), the prompt MUST explicitly instruct the task to deliver its result by calling the send_session_message tool. Without this instruction the user will receive nothing on success. \
+        \
         IMPORTANT - Target Configuration: \
         For agent_task and silent_agent_task, the target.channel and target.chat_id determine where notifications are sent. \
         - If target is omitted or fields are empty, they are automatically filled from the current conversation context. \