Replace branch-push trigger with tag-push trigger (20* pattern).
Version is extracted from the git tag itself instead of auto-generated.
Docker images are tagged with the CalVer version + floating "production" tag.
To release: git tag YYYY.M.BUILD && git push --tags
Also updates CLAUDE.md to document the new workflow and removes all
next-release branch references (branch retired).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>