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/app.dart

@@ -166,6 +166,7 @@ class _RootShellState extends State<_RootShell> {
                             const ClidePalette(),
                             const QuickOpenOverlay(),
                             const Positioned.fill(child: _WelcomeOverlay()),
+                            const ToastOverlay(),
                           ],
                         ),
                       ),