Prior REQUEST_CHANGES on head A no longer blocks formal re-review on
head B of the same open PR. Locks store reviewed head SHA, hard-stop and
mark_ready/submit gates allow a fresh decision boundary when the live
head differs, and same-head duplicates remain fail-closed. Open-PR lock
cleanup stays forbidden (#594); assessment reports locked vs current
head and stale_by_head / fresh_review_on_current_head_allowed.
Closes#620
