refactor(cmd): make git diff mockable in pr-describe command and tests

- Introduce `gitDiff` variable as a mockable wrapper for `git.Diff`.
- Update tests to use `withMockGitDiff` for injecting diff mocks.
- Minor flag description tweak for clarity.

cmd/prdescribe.go

@@ -10,6 +10,9 @@ import (
 	"gmgauthier.com/grokkit/internal/git"
 )
 
+// gitDiff is the mockable entry point (exactly like gitRun used elsewhere).
+var gitDiff = git.Diff
+
 var prDescribeCmd = &cobra.Command{
 	Use:   "pr-describe",
 	Short: "Generate full PR description from current branch",
@@ -17,16 +20,16 @@ var prDescribeCmd = &cobra.Command{
 }
 
 func init() {
-	prDescribeCmd.Flags().StringP("base", "b", "master", "Base branch to compare against")
+	prDescribeCmd.Flags().StringP("base", "b", "master", "Base branch to compare against (default: master)")
 }
 
 func runPRDescribe(cmd *cobra.Command, _ []string) {
 	base, _ := cmd.Flags().GetString("base")
 
-	// Prefer a local base, fallback to origin/<base>. Tolerant of exit code 1.
-	diff, err := git.Diff([]string{"diff", fmt.Sprintf("%s..HEAD", base), "--no-color"})
+	// Prefer local base, fallback to origin/<base>.
+	diff, err := gitDiff([]string{"diff", fmt.Sprintf("%s..HEAD", base), "--no-color"})
 	if err != nil || strings.TrimSpace(diff) == "" {
-		diff, err = git.Diff([]string{"diff", fmt.Sprintf("origin/%s..HEAD", base), "--no-color"})
+		diff, err = gitDiff([]string{"diff", fmt.Sprintf("origin/%s..HEAD", base), "--no-color"})
 		if err != nil {
 			color.Red("Failed to get branch diff: %v", err)
 			return

cmd/run_test.go

@@ -47,6 +47,13 @@ func withMockGit(fn func([]string) (string, error)) func() {
 	return func() { gitRun = orig }
 }
 
+// withMockGitDiff injects a fake for the new git.Diff (mockable var).
+func withMockGitDiff(fn func([]string) (string, error)) func() {
+	orig := gitDiff
+	gitDiff = fn
+	return func() { gitDiff = orig }
+}
+
 // testCmd returns a minimal cobra command with common flags registered.
 func testCmd() *cobra.Command {
 	c := &cobra.Command{}
@@ -260,7 +267,7 @@ func TestRunPRDescribe(t *testing.T) {
 	t.Run("no changes on branch — skips AI", func(t *testing.T) {
 		mock := &mockStreamer{}
 		defer withMockClient(mock)()
-		defer withMockGit(func(args []string) (string, error) {
+		defer withMockGitDiff(func(args []string) (string, error) {
 			return "", nil // both diff calls return empty
 		})()
 
@@ -275,7 +282,7 @@ func TestRunPRDescribe(t *testing.T) {
 		mock := &mockStreamer{response: "## PR Title\n\nDescription"}
 		defer withMockClient(mock)()
 		callCount := 0
-		defer withMockGit(func(args []string) (string, error) {
+		defer withMockGitDiff(func(args []string) (string, error) {
 			callCount++
 			if callCount == 1 {
 				return "diff --git a/foo.go b/foo.go", nil
@@ -294,7 +301,7 @@ func TestRunPRDescribe(t *testing.T) {
 		mock := &mockStreamer{response: "PR description"}
 		defer withMockClient(mock)()
 		callCount := 0
-		defer withMockGit(func(args []string) (string, error) {
+		defer withMockGitDiff(func(args []string) (string, error) {
 			callCount++
 			if callCount == 2 {
 				return "diff --git a/bar.go b/bar.go", nil
@@ -313,7 +320,7 @@ func TestRunPRDescribe(t *testing.T) {
 		mock := &mockStreamer{response: "PR description"}
 		defer withMockClient(mock)()
 		var capturedArgs []string
-		defer withMockGit(func(args []string) (string, error) {
+		defer withMockGitDiff(func(args []string) (string, error) {
 			capturedArgs = args
 			return "diff content", nil
 		})()
@@ -345,7 +352,7 @@ func TestRunPRDescribe(t *testing.T) {
 		mock := &mockStreamer{response: "PR description"}
 		defer withMockClient(mock)()
 		var capturedArgs []string
-		defer withMockGit(func(args []string) (string, error) {
+		defer withMockGitDiff(func(args []string) (string, error) {
 			capturedArgs = args
 			return "diff content", nil
 		})()