Release notes come from commit titles, not CHANGELOG.md

`softprops/action-gh-release` runs with `generate_release_notes: true`, so every release page is a list of commit titles. Meanwhile `CHANGELOG.md` is maintained by hand, grouped into "Main changes" and "Other changes", and never reaches users.

- [ ] Extract the section for the tag being built from `CHANGELOG.md`
- [ ] Pass it as the release `body`, keeping `generate_release_notes` off or appended below
- [ ] Decide the failure mode when the changelog has no section for the tag — fail the release, or fall back to generated notes

Related: #27, since a script that already parses the tag is the natural place to resolve which changelog section to read.