add toast notification system (T-50)

Non-modal operation-feedback toasts, bottom-right: a ClideToast card per
severity (success/warning/error/info), auto-dismiss (errors linger), queue
with a visible cap, slide+fade in, manual dismiss, live-region a11y.

ToastService is a MessageBus consumer — components raise a toast by publishing
to the 'toast' channel (publishToast helper), so emitters stay decoupled from
the UI. GitController's push/pull are the first emitters. ToastOverlay mounts
in the app-root Stack.

Also adds comprehensive GitController coverage: importing it for the toast
emitter test first pulled the whole file into the coverage denominator, so the
controller is now tested end to end (status/stage/commit/stash/push/pull).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/builtin/git/src/git_panel_view.dart

@@ -35,7 +35,7 @@ class _GitPanelViewState extends State<GitPanelView> {
     super.didChangeDependencies();
     if (_controller != null) return;
     final kernel = ClideKernel.of(context);
-    _controller = GitController(ipc: kernel.ipc, events: kernel.events);
+    _controller = GitController(ipc: kernel.ipc, events: kernel.events, messages: kernel.messages);
     unawaited(_controller!.load());
   }