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>

.releaser.gitlab-ci.yml

@@ -41,9 +41,8 @@
     - git config user.name  "${GITLAB_USER_NAME:-Releaser CI}"
 
   script:
-    - releaser
-        --branch "$CI_COMMIT_BRANCH"
-        $RELEASER_EXTRA_ARGS
+    # No --branch needed: releaser falls back to CI_COMMIT_BRANCH on detached HEAD.
+    - releaser $RELEASER_EXTRA_ARGS
 
   artifacts:
     reports: