PR session shows stale changed-files list after force-push

PR session shows stale changed-files list after force-push

When reviewing a PR in a worktree-backed session, the file list shown in the app (pr_changed_files in session context) is not updated after the PR branch is force-pushed.

This causes:

- The app UI continues to show files that are no longer part of the PR diff (e.g., a file removed via force-push still appears in the changed files panel).
- The agent receives stale pr_changed_files metadata, leading to incorrect file counts and failed review comment attempts on files no longer in the diff.
- git pull reports "Already up to date" because the local worktree already matches the remote HEAD — the issue is purely in the app's cached PR metadata.

Repro:

- Open a PR session for a PR with N files changed.
- On GitHub (or via CLI outside the session), force-push (not sure if force is required) the PR branch removing one of the changed files.
- Observe the app still shows N files instead of N-1.
Expected: The app should refresh PR metadata (changed files, diff stat) when the branch HEAD changes, or provide a manual "refresh" action.

---
| Field | Value |
| --- | --- |
| App version | 1.0.18 |
| OS | Windows 10.0.26310 |
| Theme | GitHub |
| Path | /chat |
| Tenure | Week 3 |

Start at the /chat worktree-backed session flow and trace where pr_changed_files and PR diff metadata are populated or cached. Reproduce the force-push scenario, then verify that the changed-files list and diff statistics update when the branch HEAD changes or through a manual refresh, without retaining removed files.

索引モデルが issue の本文から書いたものです。