Replace branch/PR-triggered Docker builds with tag-only triggers.
Images are now built only when a CalVer tag is pushed
(git tag YYYY.M.BUILD && git push origin YYYY.M.BUILD).
- Remove calver, docker-tags, and gitea-tag reusable actions
- Add inline version extraction from tag ref
- Add build cache (was missing)
- Update CLAUDE.md: remove stale next-release release workflow
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>