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>

internal/gitutil/gitutil.go

@@ -196,15 +196,17 @@ func AuthorFromConfig(repo *gogit.Repository) (name, email string) {
 	return
 }
 
-// CommitFile stages filePath (relative to worktree root) and creates a commit.
-func CommitFile(repo *gogit.Repository, filePath, message, authorName, authorEmail string) (plumbing.Hash, error) {
+// CommitFiles stages all filePaths (relative to worktree root) and creates a commit.
+func CommitFiles(repo *gogit.Repository, filePaths []string, message, authorName, authorEmail string) (plumbing.Hash, error) {
 	w, err := repo.Worktree()
 	if err != nil {
 		return plumbing.ZeroHash, err
 	}
 
-	if _, err := w.Add(filePath); err != nil {
-		return plumbing.ZeroHash, fmt.Errorf("git add %s: %w", filePath, err)
+	for _, p := range filePaths {
+		if _, err := w.Add(p); err != nil {
+			return plumbing.ZeroHash, fmt.Errorf("git add %s: %w", p, err)
+		}
 	}
 
 	hash, err := w.Commit(message, &gogit.CommitOptions{
@@ -220,6 +222,11 @@ func CommitFile(repo *gogit.Repository, filePath, message, authorName, authorEma
 	return hash, nil
 }
 
+// CommitFile stages a single file and creates a commit.
+func CommitFile(repo *gogit.Repository, filePath, message, authorName, authorEmail string) (plumbing.Hash, error) {
+	return CommitFiles(repo, []string{filePath}, message, authorName, authorEmail)
+}
+
 // CreateTag creates a lightweight tag on HEAD.
 func CreateTag(repo *gogit.Repository, tagName string) error {
 	head, err := repo.Head()