drop @immutable from events/types.dart

The prior commit swapped Flutter→meta to keep dart-test compiling, but
`meta` isn't a direct dep and `depend_on_referenced_packages` rightly
flags the import. Adding meta as a top-level dependency would violate
the prefer-zero-deps policy for what is purely a linter hint — the
classes are already const with final fields, so dropping @immutable
costs nothing at runtime and the analyzer never complained when the
annotation was absent.

Co-Authored-By: Claude <noreply@anthropic.com>

lib/kernel/src/events/types.dart

@@ -1,6 +1,3 @@
-import 'package:meta/meta.dart';
-
-@immutable
 abstract class ClideEvent {
   const ClideEvent();
 
@@ -10,7 +7,6 @@ abstract class ClideEvent {
   Map<String, Object?> payload() => const {};
 }
 
-@immutable
 class ClideEventEnvelope {
   const ClideEventEnvelope(this.event, this.timestamp);