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/semver/semver_test.go

@@ -0,0 +1,130 @@
+package semver_test
+
+import (
+	"testing"
+
+	"github.com/thokra/stamp/internal/changeset"
+	"github.com/thokra/stamp/internal/semver"
+)
+
+func TestBump_Major(t *testing.T) {
+	got, err := semver.Bump("1.2.3", changeset.BumpMajor, "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got != "2.0.0" {
+		t.Errorf("expected 2.0.0, got %s", got)
+	}
+}
+
+func TestBump_Minor(t *testing.T) {
+	got, err := semver.Bump("1.2.3", changeset.BumpMinor, "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got != "1.3.0" {
+		t.Errorf("expected 1.3.0, got %s", got)
+	}
+}
+
+func TestBump_Patch(t *testing.T) {
+	got, err := semver.Bump("1.2.3", changeset.BumpPatch, "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got != "1.2.4" {
+		t.Errorf("expected 1.2.4, got %s", got)
+	}
+}
+
+func TestBump_PreMajor(t *testing.T) {
+	got, err := semver.Bump("1.2.3", changeset.BumpPreMajor, "alpha")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got != "2.0.0-alpha.0" {
+		t.Errorf("expected 2.0.0-alpha.0, got %s", got)
+	}
+}
+
+func TestBump_PreMinor(t *testing.T) {
+	got, err := semver.Bump("1.2.3", changeset.BumpPreMinor, "beta")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got != "1.3.0-beta.0" {
+		t.Errorf("expected 1.3.0-beta.0, got %s", got)
+	}
+}
+
+func TestBump_PrePatch(t *testing.T) {
+	got, err := semver.Bump("1.2.3", changeset.BumpPrePatch, "rc")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got != "1.2.4-rc.0" {
+		t.Errorf("expected 1.2.4-rc.0, got %s", got)
+	}
+}
+
+func TestBump_PreRelease_Increment(t *testing.T) {
+	got, err := semver.Bump("1.2.4-rc.0", changeset.BumpPreRelease, "rc")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got != "1.2.4-rc.1" {
+		t.Errorf("expected 1.2.4-rc.1, got %s", got)
+	}
+}
+
+func TestBump_PreRelease_NewFromStable(t *testing.T) {
+	got, err := semver.Bump("1.2.3", changeset.BumpPreRelease, "alpha")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got != "1.2.4-alpha.0" {
+		t.Errorf("expected 1.2.4-alpha.0, got %s", got)
+	}
+}
+
+func TestBump_InvalidVersion(t *testing.T) {
+	_, err := semver.Bump("not-a-version", changeset.BumpPatch, "")
+	if err == nil {
+		t.Fatal("expected error for invalid version")
+	}
+}
+
+func TestHighestBump(t *testing.T) {
+	tests := []struct {
+		bumps    []changeset.BumpType
+		expected changeset.BumpType
+	}{
+		{[]changeset.BumpType{changeset.BumpPatch, changeset.BumpMinor}, changeset.BumpMinor},
+		{[]changeset.BumpType{changeset.BumpMinor, changeset.BumpMajor}, changeset.BumpMajor},
+		{[]changeset.BumpType{changeset.BumpPreRelease}, changeset.BumpPreRelease},
+		{[]changeset.BumpType{changeset.BumpPreRelease, changeset.BumpPatch}, changeset.BumpPatch},
+	}
+
+	for _, tt := range tests {
+		got := semver.HighestBump(tt.bumps)
+		if got != tt.expected {
+			t.Errorf("HighestBump(%v) = %s, want %s", tt.bumps, got, tt.expected)
+		}
+	}
+}
+
+func TestProjectBumps(t *testing.T) {
+	changesets := []*changeset.Changeset{
+		{Slug: "a", Bumps: map[string]changeset.BumpType{"app": changeset.BumpMinor, "lib": changeset.BumpPatch}},
+		{Slug: "b", Bumps: map[string]changeset.BumpType{"app": changeset.BumpPatch}},
+	}
+
+	bumps := semver.ProjectBumps(changesets)
+
+	if len(bumps["app"]) != 2 {
+		t.Errorf("expected 2 bumps for app, got %d", len(bumps["app"]))
+	}
+	if len(bumps["lib"]) != 1 {
+		t.Errorf("expected 1 bump for lib, got %d", len(bumps["lib"]))
+	}
+}