add Toolchain, GitClient, native directory picker

Toolchain centralizes binary resolution — replaces five ad-hoc
mechanisms (expandedPath, _resolveGit, _resolve, _resolvePtyc,
_existsOnPath). Resolves via Future.delayed after runApp to avoid
blocking the merged UI/platform thread on macOS.

GitClient wraps all git operations with a typed API. Every subprocess
call goes through _run() using toolchain.git + toolchain.gitEnv.
Replaces free functions in operations.dart.

Native directory picker: NSOpenPanel on macOS (method channel in
AppDelegate), GtkFileChooserDialog on Linux. Falls back to text-input
dialog on web or MissingPluginException. Shows "No git repo found"
dialog when the selected directory is not a git repository.

PqlClient and pane commands updated to use Toolchain. ToolCheck
replaced by Toolchain.missing/allOk. All IPC handlers now catch
GitException to prevent unhandled exceptions on the merged thread.

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

lib/kernel/src/window_controls.dart

@@ -65,4 +65,12 @@ class WindowControls extends ChangeNotifier {
       return false;
     }
   }
+
+  /// Opens the native OS directory picker.
+  /// Returns the selected path, or null if the user cancelled.
+  /// Throws [MissingPluginException] if the platform has no handler,
+  /// so callers can fall back to a text-input dialog.
+  Future<String?> pickDirectory() {
+    return _channel.invokeMethod<String>('pickDirectory');
+  }
 }