fix(cli): friendly error when publishing from a repo with no commits

Previously a brand-new repo (git init, no commits) surfaced `git stash
create: exit status 128: You do not have the initial commit yet` from
deep inside the archive helper. Now the publish flow detects this case
via `git rev-parse --verify HEAD` up front and prints "no commits in
repository; run `git add . && git commit` before publishing". Also
updates the init flow's hint to mention `git init && git commit` so
users aren't misled into thinking `git init` alone is enough.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

cmd/ninja/cmd/plugin.go

@@ -109,7 +109,7 @@ func newPluginInitCmd() *cobra.Command {
 					fmt.Fprintf(os.Stderr, "warning: could not commit plugin.mod: %v\n", err)
 				}
 			} else {
-				fmt.Println("Not in a git repo - run `git init` before `ninja plugin publish`")
+				fmt.Println("Not in a git repo - run `git init && git commit` before `ninja plugin publish`")
 			}
 			return nil
 		},
@@ -294,6 +294,10 @@ func newPluginPublishCmd() *cobra.Command {
 				return fmt.Errorf("plugin.mod must have scope, name, and version")
 			}
 
+			if err := checkRepoHasHEAD("."); err != nil {
+				return err
+			}
+
 			if !allowDirty {
 				out, _ := exec.Command("git", "status", "--porcelain").Output()
 				if len(strings.TrimSpace(string(out))) > 0 {
@@ -548,6 +552,20 @@ func runCmd(name string, args ...string) error {
 	return c.Run()
 }
 
+// checkRepoHasHEAD returns a friendlier error than the raw git failure when
+// the publish flow is invoked in a freshly-initialised repository that has
+// no commits yet. Without this, `git stash create` reports "You do not have
+// the initial commit yet" via exit status 128, which surfaces as an internal
+// failure to the user.
+func checkRepoHasHEAD(repoDir string) error {
+	cmd := exec.Command("git", "rev-parse", "--verify", "HEAD")
+	cmd.Dir = repoDir
+	if err := cmd.Run(); err != nil {
+		return fmt.Errorf("no commits in repository; run `git add . && git commit` before publishing")
+	}
+	return nil
+}
+
 func upsertPluginMod(scope, name, kind string, categories []string) error {
 	const file = "plugin.mod"
 	existing, _ := os.ReadFile(file)

cmd/ninja/cmd/plugin_test.go

@@ -0,0 +1,55 @@
+package cmd
+
+import (
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+	"testing"
+)
+
+func TestCheckRepoHasHEAD_NoCommitsReturnsFriendlyError(t *testing.T) {
+	dir := t.TempDir()
+	runGit(t, dir, "init", "-q")
+
+	err := checkRepoHasHEAD(dir)
+	if err == nil {
+		t.Fatal("expected error for repo with no commits, got nil")
+	}
+	if !strings.Contains(err.Error(), "no commits in repository") {
+		t.Errorf("error %q should mention 'no commits in repository'", err.Error())
+	}
+	if !strings.Contains(err.Error(), "git commit") {
+		t.Errorf("error %q should suggest `git commit`", err.Error())
+	}
+}
+
+func TestCheckRepoHasHEAD_WithCommitReturnsNil(t *testing.T) {
+	dir := t.TempDir()
+	runGit(t, dir, "init", "-q")
+	if err := os.WriteFile(filepath.Join(dir, "f"), []byte("hi"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	runGit(t, dir, "add", "f")
+	runGit(t, dir, "commit", "-qm", "init")
+
+	if err := checkRepoHasHEAD(dir); err != nil {
+		t.Errorf("expected nil for repo with a commit, got %v", err)
+	}
+}
+
+func runGit(t *testing.T, dir string, args ...string) {
+	t.Helper()
+	cmd := exec.Command("git", args...)
+	cmd.Dir = dir
+	cmd.Env = append(os.Environ(),
+		"GIT_AUTHOR_NAME=t",
+		"GIT_AUTHOR_EMAIL=t@t",
+		"GIT_COMMITTER_NAME=t",
+		"GIT_COMMITTER_EMAIL=t@t",
+	)
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("git %v: %v\n%s", args, err, out)
+	}
+}