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/src/daemon/pane_commands.dart

@@ -15,10 +15,11 @@ import '../ipc/envelope.dart';
 import '../ipc/schema_v1.dart';
 import '../panes/pane.dart';
 import '../panes/registry.dart';
+import '../../kernel/src/toolchain.dart';
 import 'dispatcher.dart';
 
-void registerPaneCommands(DaemonDispatcher d, PaneRegistry registry, {String defaultPtycPath = 'ptyc'}) {
-  d.register('pane.spawn', (req) => _spawn(req, registry, defaultPtycPath));
+void registerPaneCommands(DaemonDispatcher d, PaneRegistry registry, {required Toolchain toolchain}) {
+  d.register('pane.spawn', (req) => _spawn(req, registry, toolchain));
   d.register('pane.list', (req) => _list(req, registry));
   d.register('pane.close', (req) => _close(req, registry));
   d.register('pane.write', (req) => _write(req, registry));
@@ -47,7 +48,14 @@ IpcResponse _notFound(String id, String message) => IpcResponse.err(
       ),
     );
 
-Future<IpcResponse> _spawn(IpcRequest req, PaneRegistry registry, String defaultPtycPath) async {
+Future<IpcResponse> _spawn(IpcRequest req, PaneRegistry registry, Toolchain toolchain) async {
+  // Wait for toolchain resolution if it hasn't completed yet.
+  if (!toolchain.resolved) {
+    await Future.any([
+      toolchain.waitForResolution(),
+      Future.delayed(const Duration(seconds: 5)),
+    ]);
+  }
   final args = req.args;
   final rawArgv = args['argv'];
   if (rawArgv is! List || rawArgv.isEmpty) {
@@ -84,7 +92,7 @@ Future<IpcResponse> _spawn(IpcRequest req, PaneRegistry registry, String default
       cols: (args['cols'] as num?)?.toInt() ?? 80,
       rows: (args['rows'] as num?)?.toInt() ?? 24,
       title: args['title'] as String?,
-      ptycPath: (args['ptyc_path'] as String?) ?? defaultPtycPath,
+      ptycPath: (args['ptyc_path'] as String?) ?? toolchain.ptyc,
     );
     return IpcResponse.ok(id: req.id, data: pane.toJson());
   } catch (e) {