feat: Validate running inside git work tree

Ensure the application only runs if executed within a Git
work tree.

This prevents cryptic errors when attempting Git operations
in an incorrect environment, failing fast on startup.

Author: rm-hull

internal/app/app.go

@@ -16,6 +16,7 @@ import (
 var ErrAborted = errors.New("aborted")
 
 type GitClient interface {
+	IsInWorkTree() error
 	Diff() (string, error)
 	Commit(message string) error
 }
@@ -50,6 +51,11 @@ func NewApp(provider llmprovider.Provider, git GitClient, ui UIClient, prompt st
 }
 
 func (app *App) Run(ctx context.Context, userMessage string) error {
+
+	if err := app.git.IsInWorkTree(); err != nil {
+		return err
+	}
+
 	app.ui.StartSpinner(" <magenta>Running git diff</>")
 	defer app.ui.StopSpinner()
 

internal/app/app_test.go

@@ -21,8 +21,13 @@ func (m *mockProvider) Model() string {
 }
 
 type mockGitClient struct {
-	DiffFunc   func() (string, error)
-	CommitFunc func(message string) error
+	IsInWorkTreeFunc func() error
+	DiffFunc         func() (string, error)
+	CommitFunc       func(message string) error
+}
+
+func (m *mockGitClient) IsInWorkTree() error {
+	return m.IsInWorkTreeFunc()
 }
 
 func (m *mockGitClient) Diff() (string, error) {
@@ -79,10 +84,27 @@ func TestNewApp(t *testing.T) {
 func TestAppRun(t *testing.T) {
 	ctx := context.Background()
 
+	t.Run("NotInWorkTree", func(t *testing.T) {
+		mp := &mockProvider{modelName: "test-model"}
+		gitClient := &mockGitClient{
+			IsInWorkTreeFunc: func() error { return assert.AnError },
+		}
+		uiClient := &mockUIClient{
+			StartSpinnerFunc:  func(message string) {},
+			UpdateSpinnerFunc: func(message string) {},
+			StopSpinnerFunc:   func() {},
+		}
+		app := NewApp(mp, gitClient, uiClient, "prompt")
+		err := app.Run(ctx, "")
+		assert.Error(t, err)
+		assert.Equal(t, assert.AnError, err)
+	})
+
 	t.Run("DiffError", func(t *testing.T) {
 		mp := &mockProvider{modelName: "test-model"}
 		gitClient := &mockGitClient{
-			DiffFunc: func() (string, error) { return "", assert.AnError },
+			IsInWorkTreeFunc: func() error { return nil },
+			DiffFunc:         func() (string, error) { return "", assert.AnError },
 		}
 		uiClient := &mockUIClient{
 			StartSpinnerFunc:  func(message string) {},
@@ -98,7 +120,8 @@ func TestAppRun(t *testing.T) {
 	t.Run("NoStagedChanges", func(t *testing.T) {
 		mp := &mockProvider{modelName: "test-model"}
 		gitClient := &mockGitClient{
-			DiffFunc: func() (string, error) { return "", nil },
+			IsInWorkTreeFunc: func() error { return nil },
+			DiffFunc:         func() (string, error) { return "", nil },
 		}
 		uiClient := &mockUIClient{
 			StartSpinnerFunc:  func(message string) {},
@@ -117,7 +140,8 @@ func TestAppRun(t *testing.T) {
 			callFunc:  func(ctx context.Context, systemPrompt, userPrompt string) (string, error) { return "", assert.AnError },
 		}
 		gitClient := &mockGitClient{
-			DiffFunc: func() (string, error) { return "diff output", nil },
+			IsInWorkTreeFunc: func() error { return nil },
+			DiffFunc:         func() (string, error) { return "diff output", nil },
 		}
 		uiClient := &mockUIClient{
 			StartSpinnerFunc:  func(message string) {},
@@ -136,7 +160,8 @@ func TestAppRun(t *testing.T) {
 			callFunc:  func(ctx context.Context, systemPrompt, userPrompt string) (string, error) { return "llm message", nil },
 		}
 		gitClient := &mockGitClient{
-			DiffFunc: func() (string, error) { return "diff output", nil },
+			IsInWorkTreeFunc: func() error { return nil },
+			DiffFunc:         func() (string, error) { return "diff output", nil },
 		}
 		uiClient := &mockUIClient{
 			TextAreaFunc:      func(value string) (string, bool, error) { return "", false, assert.AnError },
@@ -156,7 +181,8 @@ func TestAppRun(t *testing.T) {
 			callFunc:  func(ctx context.Context, systemPrompt, userPrompt string) (string, error) { return "llm message", nil },
 		}
 		gitClient := &mockGitClient{
-			DiffFunc: func() (string, error) { return "diff output", nil },
+			IsInWorkTreeFunc: func() error { return nil },
+			DiffFunc:         func() (string, error) { return "diff output", nil },
 		}
 		uiClient := &mockUIClient{
 			TextAreaFunc:      func(value string) (string, bool, error) { return "", false, nil },
@@ -175,8 +201,9 @@ func TestAppRun(t *testing.T) {
 			callFunc:  func(ctx context.Context, systemPrompt, userPrompt string) (string, error) { return "llm message", nil },
 		}
 		gitClient := &mockGitClient{
-			DiffFunc:   func() (string, error) { return "diff output", nil },
-			CommitFunc: func(message string) error { return assert.AnError },
+			IsInWorkTreeFunc: func() error { return nil },
+			DiffFunc:         func() (string, error) { return "diff output", nil },
+			CommitFunc:       func(message string) error { return assert.AnError },
 		}
 		uiClient := &mockUIClient{
 			TextAreaFunc:      func(value string) (string, bool, error) { return "edited message", true, nil },
@@ -196,8 +223,9 @@ func TestAppRun(t *testing.T) {
 			callFunc:  func(ctx context.Context, systemPrompt, userPrompt string) (string, error) { return "llm message", nil },
 		}
 		gitClient := &mockGitClient{
-			DiffFunc:   func() (string, error) { return "diff output", nil },
-			CommitFunc: func(message string) error { return nil },
+			IsInWorkTreeFunc: func() error { return nil },
+			DiffFunc:         func() (string, error) { return "diff output", nil },
+			CommitFunc:       func(message string) error { return nil },
 		}
 		uiClient := &mockUIClient{
 			TextAreaFunc:      func(value string) (string, bool, error) { return "edited message", true, nil },

internal/git/client.go

@@ -1,9 +1,11 @@
 package git
 
 import (
+	"errors"
 	"fmt"
 	"os"
 	"os/exec"
+	"strings"
 )
 
 type Client struct{}
@@ -12,6 +14,26 @@ func NewClient() *Client {
 	return &Client{}
 }
 
+func (c *Client) IsInWorkTree() error {
+	result, err := exec.Command(
+		"git",
+		"rev-parse",
+		"--is-inside-work-tree",
+	).CombinedOutput()
+	output := strings.Trim(string(result), "\n")
+
+	if err != nil {
+		fmt.Println(output)
+		return fmt.Errorf("git rev-parse failed: %w", err)
+	}
+
+	if output != "true" {
+		return errors.New(output)
+	}
+
+	return nil
+}
+
 func (c *Client) Diff() (string, error) {
 	result, err := exec.Command(
 		"git",