BUG agent session creates PR that doesn't get linked properly

### Short summary

PR not linked to session

### Affected version or release

v0.2.5

### Installation context

_No response_

### What happened?

This happens to me on every other PR. Very annoying for obvious reasons.

The MCP function the agent tries to use to open the PR fails with 422, apparently because we work off of forks and can't create branches on the main repo. Then, it usually asks me if it is allowed to use `gh` to open the PR, and I approve. But it's about 50-50 on whether the PR actually gets linked. I've had some sessions try to update whatever local DB state is missing the linkage but that never helped either.

Happy to provide examples for investigation if it helps.

### Steps to reproduce

_No response_

### Expected behavior

PR gets linked

### Additional context

Happy to provide IDs if you tell me how to retrieve them.

影響を受ける例からセッション ID と PR ID を収集することから始め、フォークから作業する場合に MCP 422 パスと承認済みの gh フォールバックを比較します。MCP ブランチの作成に失敗した場合も含め、作成された PR が常にそのエージェントセッションにリンクされれば完了です。

索引モデルが issue の本文から書いたものです。