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/clide.dart

@@ -21,6 +21,7 @@ export 'src/editor/buffer.dart';
 export 'src/files/ignore.dart';
 export 'src/files/listing.dart' show FileEntry, listDir;
 export 'src/git/diff.dart' show GitDiff, GitHunk, DiffLine, DiffLineKind;
+export 'src/git/client.dart' show GitClient;
 export 'src/git/operations.dart' show GitLogEntry, GitException;
 export 'src/git/status.dart'
     show GitStatus, GitFileStatus, GitFileState, GitConflictType;