feat(releaser): CHANGELOG auto-update, --init, and --changelog-file flags

- Automatically write/update CHANGELOG.md on every release, grouped by
  Breaking Changes / Added / Fixed; file is created if missing and the
  new section is committed alongside pom.xml in the release commit
- Add --init flag: scaffolds a default .releaser.yml in the repo root
- Add --changelog-file flag: override the default CHANGELOG.md path
- Add CommitFiles() to gitutil so pom.xml and CHANGELOG.md are staged
  in a single commit
- Fix HTTPS push when no token is set: delegate to the git CLI so that
  system credential helpers, SSH agents, and netrc are honoured

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ROADMAP.md

@@ -55,10 +55,12 @@
 - [x] Gitea release workflow (5-platform cross-compilation, release asset upload)
 - [x] 96% test coverage with real in-memory git repos and fuzz tests for all parsers
 
-## v0.5 — Changelog
+## v0.5 — Changelog ✅
 
-- [ ] `CHANGELOG.md` generation / append (grouped by commit type)
-- [ ] `--changelog-file` flag
+- [x] `CHANGELOG.md` generation / append (grouped by commit type: Breaking Changes / Added / Fixed)
+- [x] `--changelog-file` flag to use a custom filename
+- [x] `--init` flag to scaffold a default `.releaser.yml`
+- [x] Push falls back to system `git` CLI when no token is set (uses credential helpers, SSH, netrc)
 
 ## v1.0 — Production ready