Sub-agents spawned by background agents appear in UI without user notification

### Describe the bug

When a background general-purpose agent spawns a child agent,
the child appears in the /tasks panel silently — no chat message,
no explanation of why it was started or what it does.

### Affected version

_No response_

### Steps to reproduce the behavior

1. Launch a general-purpose agent in background mode with a complex task
2. The agent internally spawns a sub-agent (e.g. to split work)
3. The sub-agent appears in /tasks panel with a different name/description
4. User has no idea who started it or why

## Actual behavior
Silent appearance in /tasks. User can only discover it via /tasks panel.

### Expected behavior

Any agent launched (directly or by another agent) should post a notification in the chat explaining what it is and who started it.

### Additional context

The parent agent also hung for 18+ minutes (7 tool_calls) with no
timeout or warning, then its child continued working after the parent
was killed.

使用重現步驟追蹤背景代理及其子代理如何為 /tasks 面板註冊，然後找出現有的聊天通知入口。完成的標準是：子代理出現時會產生一則聊天通知，說明它的作用並指出其父代理，同時涵蓋背景代理的情況。

由索引模型根據 Issue 內容生成。