SubAgent Panel with per-agent status and activity

### Feature summary

Add SubAgent Panel with per-agent status and activity

### What problem are you trying to solve?

As we create more complex agents which can spawn many subagents in parallel it's important to be able to visualise these agents, their statuses and what they are doing.
Harnesses such as Codex and OpenCode provide this type of view but it's lacking from the Copilot App which makes it hard to understand what subagents are running.

### Proposed solution

Add a sub-agents view in the right hand panel which lists all sub-agents from the session. The list should be split into active and completed sub agents. Each sub agent should show the name and their current status. Clicking on a sub agent brings up a live read-only window of their conversation with the calling agent.

### Workflow impact

This would improve the complex multi-agent workflows for all users. It would mean we can build custom "supervisor" workflows while still allowing a human to visualise what is happening in the session. It would mean users no longer need to jump ship to OpenCode or similar for these complex use cases

### Installation context

_No response_

### Additional context

_No response_

Issue ではファイル、テスト、エントリーポイントが特定されていないため、まず右側のパネルとセッションの sub-agents データフローを特定します。完了の条件は、sub-agents ビューにアクティブなエージェントと完了したエージェントが名前とステータス付きで一覧表示され、選択した各エージェントについてライブの読み取り専用会話が開くことです。

索引モデルが issue の本文から書いたものです。