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

[markfields]

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.

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