delete ToolCheck and the GraphView placeholder (T-385)

ToolCheck had zero callers. GraphView was unreachable — the graph
builtin contributes nothing, so no surface ever built it; the flat
pql-connections ListView it held was never the owned-canvas graph
anyway (T-7 cancelled). The Governance Graph idea (Q-46/Q-49) starts
fresh if it lands.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/kernel/src/tool_check.dart

@@ -1,38 +0,0 @@
-import 'dart:io';
-
-import 'package:flutter/foundation.dart';
-
-import '../../src/pty/env.dart';
-
-class ToolCheck extends ChangeNotifier {
-  bool pqlOk = false;
-  bool tmuxOk = false;
-  bool gitOk = false;
-  bool checked = false;
-
-  bool get allOk => pqlOk && tmuxOk && gitOk;
-
-  List<String> get errors => [if (!pqlOk) 'pql not found', if (!tmuxOk) 'tmux not found', if (!gitOk) 'git not found'];
-
-  /// Workspace root, set by the app at boot. Falls back to cwd.
-  static String? workspaceRoot;
-
-  Future<void> check() async {
-    pqlOk = _existsOnPath('pql');
-    tmuxOk = _existsOnPath('tmux');
-    gitOk = _existsOnPath('git');
-    checked = true;
-    notifyListeners();
-  }
-
-  /// Check if [name] exists as an executable in any PATH directory.
-  /// Uses direct file-existence checks — works inside a macOS sandbox
-  /// without needing to exec `which`.
-  static bool _existsOnPath(String name) {
-    for (final dir in expandedPath.split(':')) {
-      if (dir.isEmpty) continue;
-      if (File('$dir/$name').existsSync()) return true;
-    }
-    return false;
-  }
-}