split EventBus into DaemonBus + MessageBus

DaemonBus (was EventBus): typed events for system/IPC layer.
MessageBus: channel-based pub/sub for UI/extension coordination.
Messages carry publisher (auto-stamped from extension ID),
channel (required), timestamp, and payload. Subscribe by
publisher, channel, or both — zero collision across extensions.

Extension context gains publish() and subscribe() convenience
methods that auto-stamp the extension's ID as publisher.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/builtin/git/src/git_controller.dart

@@ -17,7 +17,7 @@ class GitController extends ChangeNotifier {
   }
 
   final DaemonClient ipc;
-  final EventBus events;
+  final DaemonBus events;
 
   StreamSubscription<DaemonEvent>? _eventSub;