persist last-opened project and wire welcome open dialog

ProjectManager.open() now saves app.lastProject to user settings.
Boot calls openLast() instead of opening cwd — first launch shows
the welcome screen, subsequent launches restore the last project
directly into Claude. The welcome "Open project" button shows a
modal path-entry dialog that validates against git and activates
the Claude tab on success. Removed the dead "New Claude session"
button since secondary sessions are spawned via command palette.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

app/lib/kernel/src/project.dart

@@ -33,11 +33,20 @@ class ProjectManager extends ChangeNotifier {
     }
     _current = Directory(root);
     await _settings.setProjectDir(_current);
+    await _settings.set<String>('app.lastProject', root);
     _events.emit(ProjectOpened(path: root));
     notifyListeners();
     return true;
   }
 
+  Future<bool> openLast() async {
+    final last = _settings.get<String>('app.lastProject');
+    if (last == null || last.isEmpty) return false;
+    final dir = Directory(last);
+    if (!await dir.exists()) return false;
+    return open(last);
+  }
+
   Future<void> close() async {
     if (_current == null) return;
     _current = null;