fix: fall back to stderr when stdout has marker but empty body

When stdout contains only a bare marker (e.g., ">") with no
review content, check stderr for the actual review instead of
returning "No review output generated".

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: wesm

internal/agent/kiro.go

@@ -166,12 +166,12 @@ func (a *KiroAgent) Review(ctx context.Context, repoPath, commitSHA, prompt stri
 	}
 
 	// Prefer the stream that contains a "> " review marker.
-	// - stdout with marker → use stdout
-	// - stdout empty → try stderr (marker or not)
+	// - stdout with marker and content → use stdout
+	// - stdout empty or marker-only → try stderr
 	// - stdout has content but no marker → use stderr only
 	//   if stderr has a marker (otherwise keep stdout)
 	result, stdoutMarker := stripKiroReview(stdout.String())
-	if !stdoutMarker {
+	if !stdoutMarker || len(result) == 0 {
 		alt, stderrMarker := stripKiroReview(stderr.String())
 		if len(alt) > 0 && (len(result) == 0 || stderrMarker) {
 			result = alt

internal/agent/kiro_test.go

@@ -379,6 +379,33 @@ func TestKiroReviewStderrFallback(t *testing.T) {
 	}
 }
 
+func TestKiroReviewStderrFallbackMarkerOnlyStdout(t *testing.T) {
+	skipIfWindows(t)
+
+	// stdout has only a bare ">" marker with no review body;
+	// stderr has the actual review. stderr should be used.
+	script := NewScriptBuilder().
+		AddRaw(`echo ">"`).
+		AddRaw(`echo "> review from stderr" >&2`).
+		Build()
+	cmdPath := writeTempCommand(t, script)
+	a := NewKiroAgent(cmdPath)
+
+	result, err := a.Review(
+		context.Background(), t.TempDir(),
+		"deadbeef", "review", nil,
+	)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if !strings.Contains(result, "review from stderr") {
+		t.Fatalf(
+			"expected stderr when stdout is marker-only, got: %q",
+			result,
+		)
+	}
+}
+
 func TestKiroReviewStderrPreferredOverStdoutNoise(t *testing.T) {
 	skipIfWindows(t)