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/kernel/src/events/bus.dart

@@ -2,8 +2,8 @@ import 'dart:async';
 
 import 'package:clide/kernel/src/events/types.dart';
 
-class EventBus {
-  EventBus();
+class DaemonBus {
+  DaemonBus();
 
   final StreamController<ClideEventEnvelope> _controller =
       StreamController<ClideEventEnvelope>.broadcast();