Demote release-note section headings to `### …`
Section headings (New/Fixed/Improved) were rendering as `<h2>` on Prowl-Site
because the markdown used `## `, the same level as the version header.
Prowl-Site's CSS targets `:global(h3)`, so the styled green monospace look
only applied to older entries that already used `### …`. Demote the canonical
form to `### New` / `### Fixed` / `### Improved`, sitting one level below the
`## [VERSION]` header that release.sh prepends.
- Backfill all 28 affected entries in CHANGELOG.md.
- Flip release-notes.sh's prompt and lint to require `### …` and reject both
`## …` and `**…**`.
- Flip release.sh's inline grep guard to the same direction.
- Companion change: 19 GitHub release bodies (v2026.3.27..v2026.4.29) were
rewritten via `gh release edit` so the public release pages match.