Moves changelog link generation from inline heading format
## [1.1.0](https://...) - date
to reference-style definitions at the bottom of the file
## [1.1.0] - date
...
[Unreleased]: https://.../src/branch/main
[1.1.0]: https://.../releases/tag/v1.1.0
This keeps headings plain, which simplifies changelog parsing (the
awk pattern in publish/action.yml now matches without special-casing
the inline URL), and follows the canonical Keep a Changelog style.
- Link Unreleased heading to repository main branch.
- Link release headings to release tag pages.
- Derive repository URL from CI metadata or origin git remote.
- Keep plain headings when repository URL cannot be resolved.
- Update tests and README usage docs for linked heading behavior.
- RecommendedTag now reads the current version from the most recent
released section heading in the changelog (## [x.y.z] - ...) when no
--version-file flag is given, removing the dependency on a separate
version file for recommendation.
- When the changelog contains no prior releases, the base version
defaults to 0.0.0, so the first recommended tag is v1.0.0 (or higher
depending on unreleased content).
- Prepare creates the release-version file if it does not already exist,
so new repositories do not need to pre-seed it.
- Add tests covering changelog-based version resolution, first-release
default, and automatic file creation.
- Update README and changelog unreleased section to document the new
behaviour.
