remove dead code and fix analyzer warnings

Unused _resolvePtycPath (bin/clide.dart), _TrafficDot (app.dart
and clide_column_hat.dart), stale @override on _spawned field
(claude_pane.dart), duplicate import and unused local in
test_app.dart, unnecessary non-null assertions in project.dart.

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

lib/kernel/src/project.dart

@@ -96,7 +96,7 @@ class ProjectManager extends ChangeNotifier {
 
     // Tell the backend isolate to (re)initialize services for this workspace.
     if (_onProjectOpen != null) {
-      await _onProjectOpen!(root);
+      await _onProjectOpen(root);
     }
 
     await _settings.setProjectDir(_current);
@@ -135,7 +135,7 @@ class ProjectManager extends ChangeNotifier {
     // the merged UI thread). Falls back to direct Process.run for tests
     // and the CLI binary.
     if (_onValidateProject != null) {
-      return _onValidateProject!(path);
+      return _onValidateProject(path);
     }
     try {
       final r = await Process.run(_toolchain.git, ['rev-parse', '--show-toplevel'], workingDirectory: path, environment: _toolchain.gitEnv);