M2: surface codex empty-output and clean CLI error messages

Addresses code-review follow-ups: codex now returns a specific error when
its -o file is empty rather than degrading to ErrEmptyResponse downstream;
CLI errors omit a dangling stderr suffix when stderr is empty.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/ai/backend.go

@@ -32,11 +32,20 @@ func (b claudeBackend) Run(ctx context.Context, prompt string) (string, error) {
 	cmd.Stdout = &out
 	cmd.Stderr = &errb
 	if err := cmd.Run(); err != nil {
-		return "", fmt.Errorf("claude: %w: %s", err, strings.TrimSpace(errb.String()))
+		return "", cmdError("claude", err, errb.String())
 	}
 	return out.String(), nil
 }
 
+// cmdError wraps a CLI failure, appending stderr only when it is non-empty so
+// the message does not end in a dangling ": ".
+func cmdError(name string, err error, stderr string) error {
+	if s := strings.TrimSpace(stderr); s != "" {
+		return fmt.Errorf("%s: %w: %s", name, err, s)
+	}
+	return fmt.Errorf("%s: %w", name, err)
+}
+
 // codexBackend shells out to `codex exec` headlessly. codex's stdout includes
 // session preamble, so the final answer is written to a temp file via -o and
 // read back. The --ignore-* and read-only flags stop it from running shell
@@ -79,12 +88,18 @@ func (b codexBackend) Run(ctx context.Context, prompt string) (string, error) {
 	var errb bytes.Buffer
 	cmd.Stderr = &errb // stdout is noisy and intentionally discarded
 	if err := cmd.Run(); err != nil {
-		return "", fmt.Errorf("codex: %w: %s", err, strings.TrimSpace(errb.String()))
+		return "", cmdError("codex", err, errb.String())
 	}
 	data, err := os.ReadFile(path)
 	if err != nil {
 		return "", fmt.Errorf("codex: read output: %w", err)
 	}
+	// A zero-exit run that wrote nothing means the -o file was never populated
+	// (e.g. flag/version skew); surface that as a codex error instead of letting
+	// it masquerade downstream as an empty model answer.
+	if len(bytes.TrimSpace(data)) == 0 {
+		return "", fmt.Errorf("codex: empty output file: %s", strings.TrimSpace(errb.String()))
+	}
 	return string(data), nil
 }