fix(baseline): record the project root relative to the baseline file

A committed baseline carried one machine's absolute path and churned whenever another checkout rewrote it. Matching never reads root.

lib/src/cli.dart

@@ -122,7 +122,11 @@ void _writeBaseline(ArgResults results, String root, List<Finding> findings) {
   final path = results['write-baseline'] as String?;
   if (path == null) return;
   try {
-    File(path).writeAsStringSync(encodeBaseline(root, findings));
+    // Record the project root relative to the baseline file, so a committed
+    // baseline does not carry one machine's absolute path (and does not churn
+    // when another checkout rewrites it). Matching never reads `root`.
+    final relativeRoot = p.relative(root, from: p.dirname(p.absolute(path)));
+    File(path).writeAsStringSync(encodeBaseline(relativeRoot, findings));
   } on FileSystemException catch (error) {
     throw PipeException(
       'cannot write baseline $path: '

test/cli_edges_test.dart

@@ -1,3 +1,4 @@
+import 'dart:convert';
 import 'dart:io';
 
 import 'package:args/command_runner.dart';
@@ -86,4 +87,31 @@ void main() {
     expect(result.stderr as String, contains('fake yams down'));
     expect(File(baseline).existsSync(), isTrue);
   });
+
+  test('a written baseline records its root relative to the file', () async {
+    // It used to store the absolute project path, so a committed baseline
+    // leaked one machine's home directory and churned on every checkout.
+    final temp = Directory.systemTemp.createTempSync('pipe-baseline-root');
+    final baseline = p.join(temp.path, 'baseline.json');
+    final result = await Process.run(Platform.resolvedExecutable, [
+      'bin/pipe.dart',
+      'check',
+      'test/fixtures/stuck_loading',
+      '--write-baseline',
+      baseline,
+      '-c',
+      '1',
+    ]);
+    expect(result.exitCode, 0, reason: '${result.stderr}');
+    final root = (jsonDecode(File(baseline).readAsStringSync()) as Map)['root'];
+    expect(p.isAbsolute(root as String), isFalse, reason: root);
+    expect(
+      p.equals(
+        p.normalize(p.join(temp.path, root)),
+        p.absolute('test/fixtures/stuck_loading'),
+      ),
+      isTrue,
+      reason: root,
+    );
+  });
 }