Review the pull request checked out in this worktree.

- Read the diff against its base (`gh pr diff`), plus enough of the surrounding
  code to judge each change in context rather than line by line.
- Look for correctness bugs, security issues, missing or weakened tests, and
  behaviour the PR description does not mention.
- Say which findings block a merge and which are preferences — do not promote a
  preference to fill out the list.
- Cite every finding by `file:line`.
