Port of box's changelog-monotonic.sh (box#122, caught in review of
box#118) as a composite action, folding in cast's port notes (cast#133).
Uniqueness on HEAD runs before anything that needs history (box#143);
containment degrades loudly, and the action defaults strict to 1 so CI
can never take the skip path.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
