fix: 任务输出回放合并活事件镜像，修复重启后 store 会话滞后导致新读取不可见

- host 半挂 ctx.on('session/event')（api-proxy 同款 API），镜像 task_output
  的 tool/call + tool/result 活事件（每会话 200 条有界环，无关结果不缓存）
- tasks.output 回放合并「store 事件日志 + 活事件镜像」，按 seq 去重排序配对
- 零 DSH 写入：会话 store 在宿主重启后可能冻结在再水合边界（持久化文件
  停在重启时刻），活事件只走 append 推送链；镜像绕开该滞后
- 测试：新增镜像用例（live 事件、跨会话隔离、种子+镜像合并不双计），
  全量 373 通过

src/context-types.ts

@@ -388,6 +388,15 @@ declare module 'cordis' {
      * {@link ./client/index.tsx}); undefined on the host side.
      */
     betterSidebar: BetterSidebarService
+    /**
+     * Subscribe to the session append feed (mirror of the cordis event API):
+     * the listener receives every appended session event with the LIVE
+     * Session instance that appended it. The api-proxy pushes the same feed
+     * to browsers; the sidebar uses it to mirror task_output events the
+     * session store's own log can lag behind (restart divergence). Returns
+     * the disposer.
+     */
+    on(event: string, listener: (session: unknown, event: SidebarSessionEvent) => void): () => void
     /**
      * Register a lifecycle callback (DSH-vendored cordis): runs at plugin
      * activation; its returned cleanup runs at disposal.