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.

首先从一个受影响的示例中收集 session ID 和 PR ID，然后在从 fork 工作时，将 MCP 422 路径与已批准的 gh fallback 进行比较。完成的标准是：即使 MCP 分支创建失败，所创建的 PR 也始终与其 agent session 关联。

由索引模型根据 Issue 内容生成。