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>

cmd/main.go

@@ -245,7 +245,7 @@ func newRootCmd() *cobra.Command {
 	root.Flags().BoolVar(&noRelease, "no-release", false, "push commit and tag but skip creating the release")
 	root.Flags().BoolVar(&noCommit, "no-commit", false, "update files but do not commit, tag, or push")
 	root.Flags().BoolVar(&tagOnly, "tag-only", false, "tag HEAD without updating files (assumes version was already committed)")
-	root.Flags().StringVar(&branchOverride, "branch", "", "override branch name detection (required in detached HEAD)")
+	root.Flags().StringVar(&branchOverride, "branch", "", "override branch name detection (detached HEAD falls back to CI_COMMIT_BRANCH, CI_COMMIT_REF_NAME, GITHUB_REF_NAME)")
 	root.Flags().StringVar(&repoPath, "repo", ".", "path to git repository")
 	root.Flags().StringVar(&pomOverride, "pom", "", "override maven.pom_path from config")
 	root.Flags().StringVar(&gradleOverride, "gradle", "", "override gradle.build_file from config")