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.go

@@ -0,0 +1,101 @@
+package changelog
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"regexp"
+	"strings"
+	"time"
+
+	"git.k3nny.fr/releaser/internal/commits"
+)
+
+var headerSubjectRe = regexp.MustCompile(`(?i)^\w+(?:\([^)]*\))?!?\s*:\s*(.+)`)
+
+// Update inserts a new release section into the CHANGELOG file at path.
+// If the file does not exist it is created with a standard header.
+// Only commits with a releasable type (fix, feat, breaking) produce bullets;
+// if none are found the file is left untouched.
+func Update(path, tag, version string, messages []string) error {
+	section := buildSection(version, messages)
+	if section == "" {
+		return nil
+	}
+
+	existing := ""
+	data, err := os.ReadFile(path)
+	if err == nil {
+		existing = string(data)
+	} else if !errors.Is(err, os.ErrNotExist) {
+		return fmt.Errorf("read %s: %w", path, err)
+	}
+
+	var out string
+	if existing == "" {
+		out = "# Changelog\n\nAll notable changes to this project will be documented in this file.\n\n" +
+			section + "\n"
+	} else {
+		// Insert above the first ## [ heading so newest release is always at top.
+		if idx := strings.Index(existing, "\n## ["); idx >= 0 {
+			out = existing[:idx+1] + section + "\n\n" + existing[idx+1:]
+		} else {
+			out = strings.TrimRight(existing, "\n") + "\n\n" + section + "\n"
+		}
+	}
+
+	return os.WriteFile(path, []byte(out), 0644)
+}
+
+func buildSection(version string, messages []string) string {
+	var breaking, feats, fixes []string
+
+	for _, msg := range messages {
+		t := commits.Parse(msg)
+		if t == commits.TypeNone {
+			continue
+		}
+		first := strings.SplitN(strings.TrimSpace(msg), "\n", 2)[0]
+		subject := extractSubject(first)
+
+		switch t {
+		case commits.TypeBreaking:
+			breaking = append(breaking, subject)
+		case commits.TypeFeat:
+			feats = append(feats, subject)
+		case commits.TypeFix:
+			fixes = append(fixes, subject)
+		}
+	}
+
+	if len(breaking)+len(feats)+len(fixes) == 0 {
+		return ""
+	}
+
+	date := time.Now().Format("2006-01-02")
+	var sb strings.Builder
+	fmt.Fprintf(&sb, "## [%s] - %s\n", version, date)
+	writeSection(&sb, "Breaking Changes", breaking)
+	writeSection(&sb, "Added", feats)
+	writeSection(&sb, "Fixed", fixes)
+
+	return strings.TrimRight(sb.String(), "\n")
+}
+
+func writeSection(sb *strings.Builder, title string, items []string) {
+	if len(items) == 0 {
+		return
+	}
+	fmt.Fprintf(sb, "\n### %s\n", title)
+	for _, item := range items {
+		fmt.Fprintf(sb, "- %s\n", item)
+	}
+}
+
+func extractSubject(header string) string {
+	m := headerSubjectRe.FindStringSubmatch(header)
+	if m != nil {
+		return strings.TrimSpace(m[1])
+	}
+	return strings.TrimSpace(header)
+}