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/config/config_test.go

@@ -0,0 +1,132 @@
+package config_test
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/thokra/stamp/internal/config"
+)
+
+const validTOML = `
+[config]
+base_branch = "main"
+changeset_dir = ".stamp"
+
+[[projects]]
+name    = "my-app"
+path    = "apps/my-app"
+version = "1.2.3"
+
+[[projects]]
+name    = "my-lib"
+path    = "libs/my-lib"
+version = "0.1.0"
+`
+
+func TestLoad_Valid(t *testing.T) {
+	dir := t.TempDir()
+	if err := os.WriteFile(filepath.Join(dir, "stamp.toml"), []byte(validTOML), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	cfg, err := config.Load(dir)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	if len(cfg.Projects) != 2 {
+		t.Fatalf("expected 2 projects, got %d", len(cfg.Projects))
+	}
+	if cfg.BaseBranch() != "main" {
+		t.Errorf("expected base_branch=main, got %s", cfg.BaseBranch())
+	}
+	if cfg.ChangesetDir() != ".stamp" {
+		t.Errorf("expected changeset_dir=.stamp, got %s", cfg.ChangesetDir())
+	}
+}
+
+func TestLoad_Defaults(t *testing.T) {
+	dir := t.TempDir()
+	minimal := "[[projects]]\nname = \"app\"\npath = \".\"\nversion = \"1.0.0\"\n"
+	if err := os.WriteFile(filepath.Join(dir, "stamp.toml"), []byte(minimal), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	cfg, err := config.Load(dir)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	if cfg.BaseBranch() != "main" {
+		t.Errorf("default base_branch should be main, got %s", cfg.BaseBranch())
+	}
+	if cfg.ChangesetDir() != ".stamp" {
+		t.Errorf("default changeset_dir should be .stamp, got %s", cfg.ChangesetDir())
+	}
+}
+
+func TestLoad_MissingFile(t *testing.T) {
+	_, err := config.Load(t.TempDir())
+	if err == nil {
+		t.Fatal("expected error for missing stamp.toml")
+	}
+}
+
+func TestLoad_NoProjects(t *testing.T) {
+	dir := t.TempDir()
+	if err := os.WriteFile(filepath.Join(dir, "stamp.toml"), []byte("[config]\n"), 0644); err != nil {
+		t.Fatal(err)
+	}
+	_, err := config.Load(dir)
+	if err == nil {
+		t.Fatal("expected error for config with no projects")
+	}
+}
+
+func TestLoad_DuplicateProjectName(t *testing.T) {
+	dir := t.TempDir()
+	dup := `
+[[projects]]
+name = "app"
+path = "."
+version = "1.0.0"
+
+[[projects]]
+name = "app"
+path = "other"
+version = "2.0.0"
+`
+	if err := os.WriteFile(filepath.Join(dir, "stamp.toml"), []byte(dup), 0644); err != nil {
+		t.Fatal(err)
+	}
+	_, err := config.Load(dir)
+	if err == nil {
+		t.Fatal("expected error for duplicate project name")
+	}
+}
+
+func TestFindProject(t *testing.T) {
+	dir := t.TempDir()
+	if err := os.WriteFile(filepath.Join(dir, "stamp.toml"), []byte(validTOML), 0644); err != nil {
+		t.Fatal(err)
+	}
+	cfg, _ := config.Load(dir)
+
+	if p := cfg.FindProject("my-app"); p == nil {
+		t.Error("expected to find my-app")
+	}
+	if p := cfg.FindProject("nonexistent"); p != nil {
+		t.Error("expected nil for nonexistent project")
+	}
+}
+
+func TestPublishDefaults(t *testing.T) {
+	var pc config.PublishConfig
+	if !pc.PublishTags() {
+		t.Error("tags should default to true")
+	}
+	if !pc.PublishReleases() {
+		t.Error("releases should default to true")
+	}
+}