feat(gitutil): release v1.9.0 — detached HEAD support in CI

- CurrentBranch falls back to CI_COMMIT_BRANCH, CI_COMMIT_REF_NAME, then
  GITHUB_REF_NAME when HEAD is detached, so --branch is no longer required
  in GitLab CI / GitHub Actions jobs
- go-git push paths (token and SSH agent) now push HEAD's commit hash to
  the branch instead of refs/heads/<branch>, which never exists in a
  detached CI checkout — go-git silently skipped the branch update and
  pushed only the tag
- .releaser.gitlab-ci.yml template drops the redundant --branch flag
- docs: README, usage, ci-integration, changelog, ROADMAP updated

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ROADMAP.md

@@ -8,7 +8,7 @@
 - [x] Commit range walker (last tag → HEAD, or full history on first release)
 - [x] Version bump calculator — returns plain `X.Y.Z` (prefix kept separate)
 - [x] `--dry-run` flag: print next version and exit
-- [x] `--branch` flag: override branch detection (detached HEAD in CI)
+- [x] `--branch` flag: override branch detection (detached HEAD in CI); detached HEAD auto-falls back to `CI_COMMIT_BRANCH` / `CI_COMMIT_REF_NAME` / `GITHUB_REF_NAME` — ✓ shipped v1.9.0, along with detached-HEAD push fix
 - [x] Exit code 2 when no releasable commits
 
 ## v0.2 — Config + Maven + local git ops ✅