v2: Event Handlers DSL

[javiertoledo]

Parent

Part of #739

Context

Event handlers currently use a class-based interface:

interface EventHandlerInterface {
  handle(event: EventInterface | NotificationInterface, register: Register): Promise<void>
}

And are stored as class instances in config:

eventHandlers: Record<EventName, Array<EventHandlerInterface>>

Proposed Changes

1. Create eventHandler() DSL function in packages/core/src/dsl/event-handler.ts
2. Update EventHandlerMetadata to use handler functions
3. Update event dispatching to call handler functions directly
4. Reimplement @EventHandler decorator as wrapper

Design Considerations

• Event handlers are linked to specific event types
• Multiple handlers can react to the same event
• Handlers receive a Register to emit further events (event chaining)
• The interface currently stores instances, not classes — the migration is straightforward

Acceptance Criteria

• eventHandler() DSL function created
• Metadata updated to function-based format
• Decorator reimplemented as wrapper
• All existing tests pass
• New tests for DSL definitions