Enforce `## New`/`## Fixed`/`## Improved` headings in release notes
The release-notes prompt previously framed section names as `**New**` /
`**Fixed**`, which is Markdown emphasis. Claude usually upgraded these to
`## ` h2 headings when there were many bullets, but for small releases
(one or two fixes) it treated `**Fixed**` as the literal output format,
producing bold-paragraph pseudo-headings that render as plain `<p><strong>`
on Prowl-Site instead of the styled `<h3>` everywhere else. Affected
2026.4.7, 4.16, 4.27, and 4.29.
- Rewrite the prompt to require literal `## New` / `## Fixed` /
`## Improved` headings and explicitly forbid `**...**` and `### ...`.
- Add a lint pass at the end of release-notes.sh that fails fast when
generated notes use a forbidden heading style.
- Add a defensive grep in release.sh before the CHANGELOG is written so
manually edited notes cannot reintroduce the bad format either.
- Backfill the four affected CHANGELOG entries to use `## ` headings.