fix: acknowledge IM tasks before debounce and queue wait

[Ivor-NCUT]

Closes #232.

Moves the existing Typing working reaction from agent-run start to accepted-message intake. This gives users immediate feedback even when:

• the 600 ms debounce window has not elapsed;
• the same scope is waiting behind a long active run;
• the global process pool delays the actual agent spawn.

The reaction promise stays non-blocking. Reactions are cleaned up after the corresponding batch finishes, including messages coalesced into one batch and messages cancelled by a slash command.

Validation:

• pnpm vitest run tests/integration/bot/markdown-stream-startup-failure.test.ts (12 passed)
• pnpm typecheck
• pnpm build
• git diff --check

[Ivor-NCUT]

补充修复了截图中的长富文本 @ 场景和启动失败反馈：

• 当 SDK 错误地给出 mentionedBot=false 时，用消息自带的结构化 mention + bot open_id/user_id 恢复识别；不使用名称或正文猜测。
• 一旦消息通过访问控制与 @ 校验，仍会在 debounce / Agent 启动之前立即添加 Typing reaction。
• Agent 进程启动失败时，固定回复：任务启动失败，请稍后重试。如果问题持续，请联系管理员检查 Agent 配置。，并移除 reaction，不泄露内部错误。
• 新增长 post mention、提前 reaction、启动失败三个回归测试。

验证：

• pnpm test -- --reporter=dot: 98 files / 626 tests passed
• pnpm typecheck
• pnpm build
• git diff --check

生产日志中的对应证据是：一条带真实 @ 的长富文本消息进入 intake 后被记录为 skip-no-mention；本补丁修复的正是该结构化 mention 被 SDK 汇总标志漏判的路径。