scripts/pr review artifacts carried no PR identity, so a review authored for
another PR validated clean and could gate someone else's landing. Stamp both
review.json and review.md with the reviewed PR number and head, anchor
pr-meta.json to the guarded pr-meta.env, and re-stamp foreign artifacts on init
instead of silently adopting them. review-claim now works in the PR worktree so
no subcommand writes review state to the shared canonical checkout.