Release v7.2.1

The pull-request rule now states the exact release-note contract, because the description is published verbatim as the release notes and agents kept publishing development write-ups instead.

Changed

• rules/pull-requests.md is the single source of the contract: an optional first ## Summary or one lead paragraph, then only ## Added, ## Changed, ## Fixed, ## Removed, ## Security, ## Deprecated; (#n) at the end of the bullet that delivers the issue, (part of #n) or see #n for one that must stay open; no Closes/Fixes/Refs lines, test plans, verification, review or provenance notes, internal status, or relative links. Reviewer information goes in a pull request comment. Includes a before/after example and a short self-check. (#444)
• ship-changes writes and re-reads the description against the contract before creating or editing a pull request, and fixes a failing verify-release-notes check by editing the description. (#444)
• The cratis-release-notes skill follows the contract and explains how to repair an already-published release. (#444)
• Statements that (#n) closes issues apply only to repositories released by cratis/release-action. (#444)
• The pull request template states the contract and no longer carries a literal example issue number. (#444)