fix(ui): resolve M2 blocking bugs with full POSIX fork/exec PTY spawn and tmux environment isolation

.agents/skills/multi-agent-mux-ui/packages/mam_pty/test/pty_runtime_test.dart

@@ -0,0 +1,27 @@
+import 'dart:async';
+import 'dart:convert';
+import 'dart:io';
+import 'package:mam_pty/mam_pty.dart';
+import 'package:test/test.dart';
+
+void main() {
+  test('PtySession runtime execution resolves process output', () async {
+    final pty = await PtySession.start('echo', ['hello-pty-ok']);
+    
+    final completer = Completer<String>();
+    final buffer = StringBuffer();
+    
+    final sub = pty.stdout.listen((data) {
+      buffer.write(utf8.decode(data));
+    }, onDone: () {
+      completer.complete(buffer.toString());
+    });
+    
+    final result = await completer.future.timeout(const Duration(seconds: 3));
+    sub.cancel();
+    pty.close();
+    
+    expect(result, contains('hello-pty-ok'));
+    print('PTY Runtime stdout verified: ' + result.trim());
+  });
+}