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/git/diff.dart

@@ -7,6 +7,8 @@ library;
 
 import 'dart:io';
 
+import 'operations.dart' show gitBin;
+
 enum DiffLineKind { context, addition, removal, header }
 
 class DiffLine {
@@ -129,7 +131,7 @@ Future<List<GitDiff>> gitDiff(
     args.addAll(paths);
   }
   final result = await Process.run(
-    'git',
+    gitBin,
     args,
     workingDirectory: workDir.path,
   );