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/main.dart

@@ -170,7 +170,7 @@ String _resolvePtyc(String repoRoot) {
 
 class _BusEventSink implements DaemonEventSink {
   _BusEventSink(this._bus);
-  final EventBus _bus;
+  final DaemonBus _bus;
 
   @override
   void emit(IpcEvent event) {