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/changeset/changeset_test.go

@@ -0,0 +1,137 @@
+package changeset_test
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/thokra/stamp/internal/changeset"
+)
+
+func TestParseYAMLFrontmatter(t *testing.T) {
+	content := `---
+bumps:
+  my-app: minor
+  my-lib: patch
+---
+
+Added a cool feature.
+
+- Detail one
+`
+	cs, err := changeset.ParseBytes("add-feature", []byte(content))
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	if cs.Slug != "add-feature" {
+		t.Errorf("expected slug=add-feature, got %s", cs.Slug)
+	}
+	if cs.Bumps["my-app"] != changeset.BumpMinor {
+		t.Errorf("expected my-app=minor, got %s", cs.Bumps["my-app"])
+	}
+	if cs.Bumps["my-lib"] != changeset.BumpPatch {
+		t.Errorf("expected my-lib=patch, got %s", cs.Bumps["my-lib"])
+	}
+	if cs.Description == "" {
+		t.Error("expected non-empty description")
+	}
+}
+
+func TestParseTOMLFrontmatter(t *testing.T) {
+	content := `+++
+[bumps]
+my-app = "major"
++++
+
+Breaking change.
+`
+	cs, err := changeset.ParseBytes("breaking", []byte(content))
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	if cs.Bumps["my-app"] != changeset.BumpMajor {
+		t.Errorf("expected my-app=major, got %s", cs.Bumps["my-app"])
+	}
+}
+
+func TestParse_InvalidBumpType(t *testing.T) {
+	content := "---\nbumps:\n  my-app: invalid\n---\ndesc\n"
+	_, err := changeset.ParseBytes("test", []byte(content))
+	if err == nil {
+		t.Fatal("expected error for invalid bump type")
+	}
+}
+
+func TestParse_MissingFrontmatter(t *testing.T) {
+	_, err := changeset.ParseBytes("test", []byte("just some text"))
+	if err == nil {
+		t.Fatal("expected error for missing frontmatter")
+	}
+}
+
+func TestParse_UnterminatedYAML(t *testing.T) {
+	content := "---\nbumps:\n  my-app: minor\n"
+	_, err := changeset.ParseBytes("test", []byte(content))
+	if err == nil {
+		t.Fatal("expected error for unterminated YAML frontmatter")
+	}
+}
+
+func TestWriteAndReadAll(t *testing.T) {
+	dir := t.TempDir()
+	bumps := map[string]changeset.BumpType{
+		"my-app": changeset.BumpMinor,
+	}
+
+	if err := changeset.Write(dir, "my-slug", bumps, "A change description"); err != nil {
+		t.Fatalf("Write failed: %v", err)
+	}
+
+	path := filepath.Join(dir, "my-slug.md")
+	if _, err := os.Stat(path); err != nil {
+		t.Fatalf("expected file to exist: %v", err)
+	}
+
+	all, err := changeset.ReadAll(dir)
+	if err != nil {
+		t.Fatalf("ReadAll failed: %v", err)
+	}
+	if len(all) != 1 {
+		t.Fatalf("expected 1 changeset, got %d", len(all))
+	}
+	if all[0].Bumps["my-app"] != changeset.BumpMinor {
+		t.Errorf("expected my-app=minor")
+	}
+}
+
+func TestDeleteAll(t *testing.T) {
+	dir := t.TempDir()
+	bumps := map[string]changeset.BumpType{"x": changeset.BumpPatch}
+	_ = changeset.Write(dir, "to-delete", bumps, "bye")
+
+	if err := changeset.DeleteAll(dir); err != nil {
+		t.Fatalf("DeleteAll failed: %v", err)
+	}
+
+	all, _ := changeset.ReadAll(dir)
+	if len(all) != 0 {
+		t.Errorf("expected 0 changesets after delete, got %d", len(all))
+	}
+}
+
+func TestGenerateSlug(t *testing.T) {
+	slugs := map[string]bool{}
+	for i := 0; i < 20; i++ {
+		s := changeset.GenerateSlug()
+		if s == "" {
+			t.Error("expected non-empty slug")
+		}
+		slugs[s] = true
+	}
+	// With 25 adjectives × 28 nouns = 700 combos, expect most to be unique across 20 runs.
+	if len(slugs) < 5 {
+		t.Errorf("expected more unique slugs, got %d unique from 20 runs", len(slugs))
+	}
+}