fix: mention 重复投递 + mail 失败通知竞态保护 + §14 设计文档同步

Bug 1: spawn_full_agent use_main_session 返回 None 导致 mention 重复投递
- 根因: use_main_session=True 时 session_id=None, return None 被 ticker
  _process_posts 误判为 spawn 失败, 每次 tick 都重试
- 修复: 引入 effective_sid = session_id or 'main', 统一用于
  _register_session / _monitor_process / return value

Bug 2: _mark_task failed 时未检查已完成状态导致误发投递失败通知
- 根因: spawner 标 failed 和 handler 标 done 竞态条件下, 已完成的
  mail task 被误发投递失败通知
- 修复: notify_mail_failed 调用前加防御性检查, 若 task 已 done 则跳过

设计文档: §13 三个 handler sections 列表同步 DeliveryChecklistSection
  及 GiteaConventionSection / WikiGuideSection, 更新 section 复用分析表
  及文件结构 section 计数

src/daemon/spawner.py

@@ -625,19 +625,24 @@ curl -X POST http://{self.api_host}:{self.api_port}/api/projects/{project_id}/ta
                 stdout=asyncio.subprocess.PIPE,
                 stderr=asyncio.subprocess.PIPE,
             )
-            self._register_session(session_id, agent_id, task_id, proc.pid,
+            # use_main_session=True 时 session_id 为 None,但 _register_session 和
+            # _monitor_process 需要一个非 None 的 key;同时 ticker 等调用方用
+            # `result is not None` 判断 spawn 是否成功,返回 None 会被误判为失败。
+            # 统一用 "main" 作为占位标识。
+            effective_sid = session_id or "main"
+            self._register_session(effective_sid, agent_id, task_id, proc.pid,
                                    broadcast_task_ids=broadcast_task_ids)
             logger.info("Spawned agent %s (session=%s, pid=%d)",
-                        agent_id, session_id, proc.pid)
+                        agent_id, effective_sid, proc.pid)
 
             # Schedule monitor(传 wrapped_on_complete)
             asyncio.create_task(
-                self._monitor_process(session_id, proc, agent_id, task_id,
+                self._monitor_process(effective_sid, proc, agent_id, task_id,
                                       on_complete=_wrapped_on_complete,
                                       db_path=task_db_path or self.db_path)
             )
 
-            return session_id
+            return effective_sid
 
         except Exception as e:
             # spawn 失败也要 release counter
@@ -1949,6 +1954,19 @@ curl -X POST http://{api_host}:{api_port}/api/projects/{project_id}/tasks/{task_
                 try:
                     from src.daemon.mail_notify import _is_mail_project, notify_mail_failed
                     if _is_mail_project(db_path):
+                        # 防御性检查：如果 task 已经 done，不触发失败通知（竞态保护）
+                        # 场景：spawner 标 failed 和 handler 标 done 同时发生
+                        try:
+                            conn2 = get_connection(db_path)
+                            current_status = conn2.execute(
+                                "SELECT status FROM tasks WHERE id=?", (task_id,)
+                            ).fetchone()
+                            conn2.close()
+                            if current_status and current_status["status"] == "done":
+                                logger.info("Task %s already done, skipping mail failure notification", task_id)
+                                return
+                        except Exception:
+                            pass
                         # Mail 失败：通知发件人，不 @pangtong
                         notify_mail_failed(db_path, task_id, reason, detail)
                     else: