feat: add git wrapper package for subprocess operations

Operations:
- Clone, Checkout, Fetch, Pull, Push
- Add, Commit, CommitAll
- Status, CurrentBranch, HasUncommittedChanges, RemoteURL
- Merge, Rebase, AbortMerge, AbortRebase
- CreateBranch, DeleteBranch, Rev, IsAncestor

Error handling:
- Detects ErrNotARepo, ErrMergeConflict, ErrAuthFailure, ErrRebaseConflict
- Wraps git stderr in error messages

Closes gt-u1j.3

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/git/git_test.go

@@ -0,0 +1,188 @@
+package git
+
+import (
+	"os"
+	"os/exec"
+	"path/filepath"
+	"testing"
+)
+
+func initTestRepo(t *testing.T) string {
+	t.Helper()
+	dir := t.TempDir()
+
+	// Initialize repo
+	cmd := exec.Command("git", "init")
+	cmd.Dir = dir
+	if err := cmd.Run(); err != nil {
+		t.Fatalf("git init: %v", err)
+	}
+
+	// Configure user for commits
+	cmd = exec.Command("git", "config", "user.email", "test@test.com")
+	cmd.Dir = dir
+	cmd.Run()
+	cmd = exec.Command("git", "config", "user.name", "Test User")
+	cmd.Dir = dir
+	cmd.Run()
+
+	// Create initial commit
+	testFile := filepath.Join(dir, "README.md")
+	if err := os.WriteFile(testFile, []byte("# Test\n"), 0644); err != nil {
+		t.Fatalf("write file: %v", err)
+	}
+	cmd = exec.Command("git", "add", ".")
+	cmd.Dir = dir
+	cmd.Run()
+	cmd = exec.Command("git", "commit", "-m", "initial")
+	cmd.Dir = dir
+	cmd.Run()
+
+	return dir
+}
+
+func TestCurrentBranch(t *testing.T) {
+	dir := initTestRepo(t)
+	g := NewGit(dir)
+
+	branch, err := g.CurrentBranch()
+	if err != nil {
+		t.Fatalf("CurrentBranch: %v", err)
+	}
+
+	// Modern git uses "main", older uses "master"
+	if branch != "main" && branch != "master" {
+		t.Errorf("branch = %q, want main or master", branch)
+	}
+}
+
+func TestStatus(t *testing.T) {
+	dir := initTestRepo(t)
+	g := NewGit(dir)
+
+	// Should be clean initially
+	status, err := g.Status()
+	if err != nil {
+		t.Fatalf("Status: %v", err)
+	}
+	if !status.Clean {
+		t.Error("expected clean status")
+	}
+
+	// Add an untracked file
+	testFile := filepath.Join(dir, "new.txt")
+	if err := os.WriteFile(testFile, []byte("new"), 0644); err != nil {
+		t.Fatalf("write file: %v", err)
+	}
+
+	status, err = g.Status()
+	if err != nil {
+		t.Fatalf("Status: %v", err)
+	}
+	if status.Clean {
+		t.Error("expected dirty status")
+	}
+	if len(status.Untracked) != 1 {
+		t.Errorf("untracked = %d, want 1", len(status.Untracked))
+	}
+}
+
+func TestAddAndCommit(t *testing.T) {
+	dir := initTestRepo(t)
+	g := NewGit(dir)
+
+	// Create a new file
+	testFile := filepath.Join(dir, "new.txt")
+	if err := os.WriteFile(testFile, []byte("new content"), 0644); err != nil {
+		t.Fatalf("write file: %v", err)
+	}
+
+	// Add and commit
+	if err := g.Add("new.txt"); err != nil {
+		t.Fatalf("Add: %v", err)
+	}
+	if err := g.Commit("add new file"); err != nil {
+		t.Fatalf("Commit: %v", err)
+	}
+
+	// Should be clean
+	status, err := g.Status()
+	if err != nil {
+		t.Fatalf("Status: %v", err)
+	}
+	if !status.Clean {
+		t.Error("expected clean after commit")
+	}
+}
+
+func TestHasUncommittedChanges(t *testing.T) {
+	dir := initTestRepo(t)
+	g := NewGit(dir)
+
+	has, err := g.HasUncommittedChanges()
+	if err != nil {
+		t.Fatalf("HasUncommittedChanges: %v", err)
+	}
+	if has {
+		t.Error("expected no changes initially")
+	}
+
+	// Modify a file
+	testFile := filepath.Join(dir, "README.md")
+	if err := os.WriteFile(testFile, []byte("modified"), 0644); err != nil {
+		t.Fatalf("write file: %v", err)
+	}
+
+	has, err = g.HasUncommittedChanges()
+	if err != nil {
+		t.Fatalf("HasUncommittedChanges: %v", err)
+	}
+	if !has {
+		t.Error("expected changes after modify")
+	}
+}
+
+func TestCheckout(t *testing.T) {
+	dir := initTestRepo(t)
+	g := NewGit(dir)
+
+	// Create a new branch
+	if err := g.CreateBranch("feature"); err != nil {
+		t.Fatalf("CreateBranch: %v", err)
+	}
+
+	// Checkout the new branch
+	if err := g.Checkout("feature"); err != nil {
+		t.Fatalf("Checkout: %v", err)
+	}
+
+	branch, _ := g.CurrentBranch()
+	if branch != "feature" {
+		t.Errorf("branch = %q, want feature", branch)
+	}
+}
+
+func TestNotARepo(t *testing.T) {
+	dir := t.TempDir() // Empty dir, not a git repo
+	g := NewGit(dir)
+
+	_, err := g.CurrentBranch()
+	if err != ErrNotARepo {
+		t.Errorf("expected ErrNotARepo, got %v", err)
+	}
+}
+
+func TestRev(t *testing.T) {
+	dir := initTestRepo(t)
+	g := NewGit(dir)
+
+	hash, err := g.Rev("HEAD")
+	if err != nil {
+		t.Fatalf("Rev: %v", err)
+	}
+
+	// Should be a 40-char hex string
+	if len(hash) != 40 {
+		t.Errorf("hash length = %d, want 40", len(hash))
+	}
+}