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>

test/kernel/src/events/bus_test.dart

@@ -2,10 +2,10 @@ import 'package:clide/kernel/kernel.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 void main() {
-  group('EventBus', () {
-    late EventBus bus;
+  group('DaemonBus', () {
+    late DaemonBus bus;
 
-    setUp(() => bus = EventBus());
+    setUp(() => bus = DaemonBus());
     tearDown(() => bus.dispose());
 
     test('emit delivers to stream subscribers', () async {