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/git_commands_test.dart

@@ -1,6 +1,8 @@
 import 'dart:io';
 
 import 'package:clide/clide.dart';
+import 'package:clide/kernel/src/toolchain.dart';
+import 'package:clide/src/git/client.dart';
 import 'package:clide/src/daemon/git_commands.dart';
 import 'package:test/test.dart';
 
@@ -32,7 +34,10 @@ void main() {
 
     sink = RecordingEventSink();
     dispatcher = DaemonDispatcher();
-    registerGitCommands(dispatcher, sandbox, sink);
+    final toolchain = Toolchain();
+    toolchain.applyResolved(Toolchain.resolvePaths(workspaceRoot: sandbox.path));
+    final gitClient = GitClient(toolchain: toolchain, workDir: sandbox);
+    registerGitCommands(dispatcher, gitClient, sink);
   });
 
   tearDown(() async {