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/tool_check.dart

@@ -2,6 +2,8 @@ import 'dart:io';
 
 import 'package:flutter/foundation.dart';
 
+import '../../src/pty/env.dart';
+
 class ToolCheck extends ChangeNotifier {
   bool ptycOk = false;
   bool pqlOk = false;
@@ -18,24 +20,31 @@ class ToolCheck extends ChangeNotifier {
         if (!gitOk) 'git not found',
       ];
 
+  /// Workspace root, set by the app at boot. Falls back to cwd.
+  static String? workspaceRoot;
+
   Future<void> check() async {
-    final cwd = Directory.current.path;
-    ptycOk = File('$cwd/native/linux-x64/ptyc').existsSync() ||
-        File('$cwd/ptyc/bin/ptyc').existsSync() ||
-        await _which('ptyc');
-    pqlOk = await _which('pql');
-    tmuxOk = await _which('tmux');
-    gitOk = await _which('git');
+    final root = workspaceRoot ?? Directory.current.path;
+    ptycOk = File('$root/native/linux-x64/ptyc').existsSync() ||
+        File('$root/native/macos-arm64/ptyc').existsSync() ||
+        File('$root/native/macos-x64/ptyc').existsSync() ||
+        File('$root/ptyc/bin/ptyc').existsSync() ||
+        _existsOnPath('ptyc');
+    pqlOk = _existsOnPath('pql');
+    tmuxOk = _existsOnPath('tmux');
+    gitOk = _existsOnPath('git');
     checked = true;
     notifyListeners();
   }
 
-  static Future<bool> _which(String name) async {
-    try {
-      final r = await Process.run('which', [name]);
-      return r.exitCode == 0;
-    } catch (_) {
-      return false;
+  /// 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;
   }
 }