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>

test/daemon/pql_commands_test.dart

@@ -1,6 +1,7 @@
 import 'dart:io';
 
 import 'package:clide/clide.dart';
+import 'package:clide/kernel/src/toolchain.dart';
 import 'package:clide/src/daemon/pql_commands.dart';
 import 'package:clide/src/pql/client.dart';
 import 'package:test/test.dart';
@@ -10,7 +11,9 @@ void main() {
   late PqlClient pql;
 
   setUp(() {
-    pql = PqlClient(workDir: Directory.current);
+    final toolchain = Toolchain();
+    toolchain.applyResolved(Toolchain.resolvePaths(workspaceRoot: Directory.current.path));
+    pql = PqlClient(workDir: Directory.current, toolchain: toolchain);
     dispatcher = DaemonDispatcher();
     registerPqlCommands(dispatcher, pql);
   });