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/status.dart

@@ -7,6 +7,8 @@ library;
 
 import 'dart:io';
 
+import 'operations.dart' show gitBin;
+
 enum GitFileState {
   added,
   modified,
@@ -110,11 +112,16 @@ class GitStatus {
 
 /// Run `git status` and parse the result.
 Future<GitStatus> gitStatus(Directory workDir) async {
-  final branchResult = await Process.run(
-    'git',
-    ['status', '--porcelain=v2', '--branch', '-z'],
-    workingDirectory: workDir.path,
-  );
+  final ProcessResult branchResult;
+  try {
+    branchResult = await Process.run(
+      gitBin,
+      ['status', '--porcelain=v2', '--branch', '-z'],
+      workingDirectory: workDir.path,
+    );
+  } on ProcessException {
+    return const GitStatus(branch: null, entries: []);
+  }
 
   String? branch;
   String? upstream;
@@ -138,11 +145,16 @@ Future<GitStatus> gitStatus(Directory workDir) async {
     }
   }
 
-  final result = await Process.run(
-    'git',
-    ['status', '--porcelain=v1', '-z'],
-    workingDirectory: workDir.path,
-  );
+  final ProcessResult result;
+  try {
+    result = await Process.run(
+      gitBin,
+      ['status', '--porcelain=v1', '-z'],
+      workingDirectory: workDir.path,
+    );
+  } on ProcessException {
+    return GitStatus(branch: branch, entries: const [], upstream: upstream, ahead: ahead, behind: behind);
+  }
 
   if (result.exitCode != 0) {
     return GitStatus(
@@ -154,7 +166,7 @@ Future<GitStatus> gitStatus(Directory workDir) async {
     );
   }
 
-  final entries = _parsePorcelainV1(result.stdout as String);
+  final entries = parsePorcelainV1(result.stdout as String);
   return GitStatus(
     branch: branch,
     upstream: upstream,
@@ -164,7 +176,7 @@ Future<GitStatus> gitStatus(Directory workDir) async {
   );
 }
 
-List<GitFileStatus> _parsePorcelainV1(String output) {
+List<GitFileStatus> parsePorcelainV1(String output) {
   if (output.isEmpty) return const [];
   final entries = <GitFileStatus>[];
   final parts = output.split('\x00');