standing PR: a notes region missing its end marker swallows the rest of the body #781

This was generated by AI during triage.

Found while reviewing #761.

Agent Brief

Category: bug
Summary: A notes region whose end marker was deleted swallows the rest of the PR or issue body, which is then preserved and published as the release notes.

Current behavior:
Region extraction treats a missing close marker as "everything after the opener", to support legacy bodies that carried only an opener. If a maintainer deletes <!-- releasekit-notes-end:<key> --> while editing:

• the extracted "edit" includes every later region, the --- rule, the merge hint and the attribution footer;
• that text is merged back as the region, duplicating it in the body on the next update;
• publish sends it as the GitHub Release body.

This affects the standing PR's notes regions (every key, and since #761 the sync monorepo key too) and the draft issue.

Desired behavior:

• A notes region without its close marker is treated as unreadable. It's regenerated from fresh notes and a warning names the key, matching what the draft flow already does when markers are removed.
• No content outside a region's own markers is ever extracted as that region.
• Releasekit is pre-1.0, so the legacy opener-only fallback can go for notes regions. Check that no other caller of the shared marker-region extraction still relies on it before changing the shared code.

Key interfaces:

• extractMarkerRegion and the notes-region extraction built on it.
• The standing-PR update and publish paths, and publishFromDraft.

Acceptance criteria:

• A standing-PR body whose notes region lacks its end marker yields no edit for that key and a warning, and the next update re-renders a well-formed region.
• Publish from such a body doesn't send footer or other-region text as a release body.
• Bodies with intact markers behave as before.

Out of scope:

• Restoring an edit whose end marker was deleted.