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/os.dart

@@ -16,12 +16,12 @@ class OsLifecycleEvent extends ClideEvent {
 }
 
 class OsBridge {
-  OsBridge({required Logger log, required EventBus events})
+  OsBridge({required Logger log, required DaemonBus events})
       : _log = log,
         _events = events;
 
   final Logger _log;
-  final EventBus _events;
+  final DaemonBus _events;
 
   Future<bool> openURL(String url) async {
     final cmd = _openCommand();