fix(cli): keep status messages off stdout so JSON output stays parseable

`check --format json --write-baseline b.json` printed "wrote baseline to ..."
after the JSON document, and `guide --yams` printed "persisted to yams as ..."
after the brief, so stdout was no longer a single JSON document (or a clean
brief) and `| jq` failed. Both status lines now go to stderr.

lib/src/cli.dart

@@ -110,7 +110,9 @@ void _writeBaseline(ArgResults results, String root, List<Finding> findings) {
   final path = results['write-baseline'] as String?;
   if (path == null) return;
   File(path).writeAsStringSync(encodeBaseline(root, findings));
-  stdout.writeln(
+  // Status goes to stderr: after `--format json` stdout is the JSON document,
+  // and a trailing status line made it unparseable.
+  stderr.writeln(
     'wrote baseline to $path (${findings.length} findings accepted)',
   );
 }
@@ -753,7 +755,7 @@ class GuideCommand extends Command<int> {
           stderr.writeln('pipe: could not persist to yams: $error');
           return 1;
         }
-        stdout.writeln('persisted to yams as $name');
+        stderr.writeln('persisted to yams as $name');
       }
       _writeBaseline(results, project.root, analysis.report.findings);
     } finally {

test/stdout_json_test.dart

@@ -0,0 +1,27 @@
+import 'dart:convert';
+import 'dart:io';
+
+import 'package:test/test.dart';
+
+void main() {
+  test('check --format json --write-baseline keeps stdout pure JSON', () async {
+    // The "wrote baseline" status line used to follow the JSON document on
+    // stdout, so `pipe check --format json --write-baseline b.json | jq` failed.
+    final temp = Directory.systemTemp.createTempSync('pipe-json-stdout');
+    final baseline = '${temp.path}/baseline.json';
+    final result = await Process.run(Platform.resolvedExecutable, [
+      'bin/pipe.dart',
+      'check',
+      'test/fixtures/stuck_loading',
+      '--format',
+      'json',
+      '--write-baseline',
+      baseline,
+      '-c',
+      '1',
+    ]);
+    expect(result.exitCode, 0, reason: '${result.stderr}');
+    expect(() => jsonDecode(result.stdout as String), returnsNormally);
+    expect(result.stderr as String, contains('wrote baseline'));
+  });
+}