feat: initial stamp implementation

- stamp add: create .stamp/*.md changeset files (interactive + --no-interactive)
- stamp status: show pending stamps and projected next versions (plain + --json)
- stamp version: consume stamps, bump semver, update CHANGELOGs, commit
- stamp publish: create git tags, GitHub/Gitea releases, upload artifacts
- stamp comment: post/update idempotent PR comments via GitHub/Gitea API
- Supports monorepos via stamp.toml [[projects]] blocks
- Changeset files support YAML (---) and TOML (+++) frontmatter
- Semver + pre-release support (alpha, beta, rc)
- Examples and workflow documentation

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

internal/changelog/changelog_test.go

@@ -0,0 +1,107 @@
+package changelog_test
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/thokra/stamp/internal/changelog"
+)
+
+var testDate = time.Date(2026, 3, 8, 0, 0, 0, 0, time.UTC)
+
+func TestAppend_NewFile(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "CHANGELOG.md")
+
+	err := changelog.Append(path, changelog.Entry{
+		Version:     "1.0.0",
+		Date:        testDate,
+		Description: "Initial release",
+	})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	data, _ := os.ReadFile(path)
+	content := string(data)
+
+	if !strings.Contains(content, "## [1.0.0] - 2026-03-08") {
+		t.Errorf("expected version header, got:\n%s", content)
+	}
+	if !strings.Contains(content, "Initial release") {
+		t.Errorf("expected description in changelog")
+	}
+}
+
+func TestAppend_ExistingFile(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "CHANGELOG.md")
+
+	// Write first version.
+	_ = changelog.Append(path, changelog.Entry{
+		Version:     "1.0.0",
+		Date:        testDate,
+		Description: "First release",
+	})
+
+	// Append second version.
+	_ = changelog.Append(path, changelog.Entry{
+		Version:     "1.1.0",
+		Date:        testDate.Add(24 * time.Hour),
+		Description: "Second release",
+	})
+
+	data, _ := os.ReadFile(path)
+	content := string(data)
+
+	if !strings.Contains(content, "## [1.0.0]") {
+		t.Error("expected 1.0.0 section")
+	}
+	if !strings.Contains(content, "## [1.1.0]") {
+		t.Error("expected 1.1.0 section")
+	}
+
+	// New version should appear before old.
+	idx110 := strings.Index(content, "## [1.1.0]")
+	idx100 := strings.Index(content, "## [1.0.0]")
+	if idx110 > idx100 {
+		t.Error("expected 1.1.0 to appear before 1.0.0 (newest first)")
+	}
+}
+
+func TestAppend_EmptyDescription(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "CHANGELOG.md")
+
+	err := changelog.Append(path, changelog.Entry{
+		Version: "2.0.0",
+		Date:    testDate,
+	})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	data, _ := os.ReadFile(path)
+	if !strings.Contains(string(data), "## [2.0.0]") {
+		t.Error("expected version header even with empty description")
+	}
+}
+
+func TestAppend_CreatesDirectory(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "sub", "dir", "CHANGELOG.md")
+
+	err := changelog.Append(path, changelog.Entry{
+		Version: "1.0.0",
+		Date:    testDate,
+	})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if _, err := os.Stat(path); err != nil {
+		t.Errorf("expected file to be created: %v", err)
+	}
+}