Context: BUILD_REVIEW.md already notes drift between reindeer.toml and
third-party/Cargo.toml as a risk. Catching it in CI is cheap.
Scope: CI job that runs reindeer buckify and fails if the working tree would change.
Fallback if --check doesn't exist: run reindeer buckify && git diff --exit-code.
Acceptance: job exists; passes on clean tree.