surface kernel notifications as toasts (T-382)

The Notifications service had zero widget consumers — anything pushed
through ctx.notify (cli_install's dogfood warnings, install results)
accumulated in a list nothing rendered. The service now takes the
kernel MessageBus and publishes each notification to the toast
channel with mapped severity, so the existing ToastOverlay renders
them; the active list stays for API compatibility. Chose routing over
building a notifications tray nobody asked for.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/kernel/src/facade.dart

@@ -166,7 +166,7 @@ class KernelServices {
     final readerNav = ReaderNavRegistry(messages);
     final clipboard = ClideClipboard();
     final files = FileServices(events);
-    final notify = Notifications();
+    final notify = Notifications(messages: messages);
     final dialog = DialogRouter();
     final tray = TrayRegistry();
     final secrets = SecretsVault();