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/changelog/changelog_test.go

@@ -0,0 +1,111 @@
+package changelog
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestUpdateNewFile(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "CHANGELOG.md")
+
+	if err := Update(path, "v1.2.0", "1.2.0", []string{
+		"feat: add widget",
+		"fix: off-by-one in parser",
+	}); err != nil {
+		t.Fatal(err)
+	}
+
+	data, _ := os.ReadFile(path)
+	s := string(data)
+	if !strings.Contains(s, "## [1.2.0]") {
+		t.Error("expected version header")
+	}
+	if !strings.Contains(s, "### Added") {
+		t.Error("expected Added section")
+	}
+	if !strings.Contains(s, "### Fixed") {
+		t.Error("expected Fixed section")
+	}
+	if !strings.Contains(s, "add widget") {
+		t.Error("expected feat subject")
+	}
+	if !strings.Contains(s, "off-by-one in parser") {
+		t.Error("expected fix subject")
+	}
+}
+
+func TestUpdateExistingFile(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "CHANGELOG.md")
+
+	// Seed with an older release.
+	os.WriteFile(path, []byte("# Changelog\n\n## [1.1.0] - 2026-01-01\n\n### Added\n- old stuff\n"), 0644)
+
+	if err := Update(path, "v1.2.0", "1.2.0", []string{"feat: new thing"}); err != nil {
+		t.Fatal(err)
+	}
+
+	data, _ := os.ReadFile(path)
+	s := string(data)
+
+	newIdx := strings.Index(s, "## [1.2.0]")
+	oldIdx := strings.Index(s, "## [1.1.0]")
+	if newIdx < 0 || oldIdx < 0 {
+		t.Fatal("both versions should appear in CHANGELOG")
+	}
+	if newIdx > oldIdx {
+		t.Error("new version should appear before old version")
+	}
+}
+
+func TestUpdateNoReleasableCommits(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "CHANGELOG.md")
+
+	if err := Update(path, "v1.0.1", "1.0.1", []string{
+		"chore: update deps",
+		"docs: fix typo",
+	}); err != nil {
+		t.Fatal(err)
+	}
+
+	// File should NOT have been created.
+	if _, err := os.Stat(path); err == nil {
+		t.Error("file should not be created when there are no releasable commits")
+	}
+}
+
+func TestUpdateBreakingSection(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "CHANGELOG.md")
+
+	if err := Update(path, "v2.0.0", "2.0.0", []string{
+		"feat!: redesign API",
+		"fix(core): nil panic",
+	}); err != nil {
+		t.Fatal(err)
+	}
+
+	data, _ := os.ReadFile(path)
+	s := string(data)
+	if !strings.Contains(s, "### Breaking Changes") {
+		t.Error("expected Breaking Changes section")
+	}
+	if !strings.Contains(s, "redesign API") {
+		t.Error("expected breaking subject")
+	}
+}
+
+func TestUpdateReadError(t *testing.T) {
+	dir := t.TempDir()
+	// Create a directory where the file should be — ReadFile will error.
+	os.Mkdir(filepath.Join(dir, "CHANGELOG.md"), 0755)
+
+	err := Update(filepath.Join(dir, "CHANGELOG.md"), "v1.0.0", "1.0.0", []string{"fix: something"})
+	if err == nil {
+		t.Error("expected error when path is a directory")
+	}
+}