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/files.dart

@@ -31,7 +31,7 @@ class FilesDropped extends ClideEvent {
 /// the service surface real.
 class FileServices {
   FileServices(this._events);
-  final EventBus _events;
+  final DaemonBus _events;
 
   Future<List<String>> pickOpen({
     List<String> extensions = const [],