Review freshness and VCS correctness

Provenance stamp: This issue was triaged on June 13, 2026 as part of the older-than-two-month GitHub issue triage exercise.

Source issues: #475, #492

## Summary

The review UI needs to stay correct when the underlying code changes or when the repository is not a straightforward git workspace. These two issues both concern correctness of the review snapshot and the VCS data behind it.

## What the source issues requested

- #475 tracked P4 support bugs, incomplete migrations, and parity gaps across Bun, Pi, and OpenCode. Some parity work appears to have improved since the issue was opened, but residual P4 correctness checks remain important.
- #492 requested detection of PR updates while a review is open so reviewers do not submit comments against a stale head commit or review an outdated diff.

## Triage determination

These belong together because both affect whether the review surface is showing the right code and submitting feedback against the right target. The replacement issue should track current VCS correctness, stale PR refresh behavior, and any residual P4 edge cases.

## Acceptance criteria

- PR review sessions detect when the PR head changes while the review is open.
- The UI shows a clear refresh prompt when the review snapshot is stale.
- Refresh updates the diff, file content, PR metadata, head SHA, and submission target.
- In-progress annotations are preserved or clearly handled during refresh.
- P4 default changelist fallback does not land users on an empty diff when numbered changelists exist.
- P4 diff conversion handles separator headers, deleted files, added files with trailing newlines, and path mapping correctly.
- Current Pi and OpenCode VCS parity is verified and documented.
- The old source issues can be closed once current residuals are tracked here.