Custom agent reverts to default; cannot set a custom agent as persistent default

### Summary

Two related issues with **custom agents** in the GitHub Copilot app, surfaced during a customer evaluation (designer-focused UI prototyping workflow, macOS):

1. **Custom agent reverts to "default agent."** After selecting a custom agent for a session, the app reverts the selection back to the default agent (observed across sessions / on relaunch).
2. **No way to set a custom agent as the persistent default.** There is no supported setting to pin a custom agent so that new sessions start with it instead of the default agent. Users must re-select the custom agent every time.

### Impact

For non-technical users (e.g. designers) this "re-select the agent every session" toil is a real adoption blocker — one of the customer's explicit requirements is *reduced manual per-session option-selecting*. Today the Sessions settings expose a default **model**, custom instructions, and auto-approve, but not a default **agent**.

### Environment

- GitHub Copilot app (agent-native desktop), macOS (Apple Silicon)
- Custom agents configured per the custom-agents configuration docs

### Expected

- A selected custom agent persists across sessions/relaunch.
- A supported way to set a custom agent as the persistent default for new sessions (parallel to the existing default-model setting).

### Notes

Filed on behalf of an account team evaluating the app against a customer's requirements. Happy to connect the reporting customer for repro details if useful.

issue に記載されている Sessions 設定と custom-agents の構成から始め、選択したエージェントが Sessions とアプリの再起動をまたいでどのように保存されるかを追跡します。Session を作成してアプリを再起動し、結果として選択される内容を確認することで、選択したカスタムエージェントの永続化と、デフォルトエージェントを選択するための設定の両方を検証します。

索引モデルが issue の本文から書いたものです。