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/editor/src/editor_controller.dart

@@ -17,14 +17,14 @@ import 'package:clide/kernel/kernel.dart';
 import 'package:flutter/foundation.dart';
 
 class EditorController extends ChangeNotifier {
-  EditorController({required this.ipc, required EventBus events})
+  EditorController({required this.ipc, required DaemonBus events})
       : _events = events {
     _eventSub = events.on<DaemonEvent>().listen(_onEvent);
   }
 
   final DaemonClient ipc;
   // ignore: unused_field  — kept for future subscription changes
-  final EventBus _events;
+  final DaemonBus _events;
 
   StreamSubscription<DaemonEvent>? _eventSub;