* Restore changelog entries for betas
Beta users check release notes, so each beta now updates an in-place
CHANGELOG.md entry that promotion overwrites with the final stable entry
— no -beta.N heading is left behind. The release machinery already
parses beta headings and syncs them; this restores the policy in the
docs and the release-beta skill, reversing the docs side of 28415b354.
* Spell out in-place beta overwrite in the promotion checklist
The stable checklist said only "update CHANGELOG.md", so a promotion
followed by the letter could stack a fresh entry on top of the
-beta.N block — the stale heading the policy forbids. Make the
promotion path overwrite-in-place explicit, matching the beta checklist.
