Run git diff @{upstream}...HEAD (or git diff main...HEAD / git diff HEAD~1
if there's no upstream) to get the unified diff under review. If there are
uncommitted changes, or the range diff is empty, also run git diff HEAD and
include the working-tree changes in scope — the review often runs before the
commit. If a PR number, branch name, or file path was passed as an argument,
review that target instead. Treat this diff as the review scope.