feat: add gateway protocol attachment support

Extends the gateway wire protocol with GatewayAttachment type and
attachment event. agent.send handler now accepts optional attachments
parameter and converts them for the agent pipeline. Includes 5 new
tests for protocol and handler layers.

src/gateway/index.ts

@@ -20,11 +20,13 @@ export type {
   GatewayResponse,
   GatewayError,
   GatewayEvent,
+  GatewayAttachment,
   OutboundMessage,
   EventType,
   ContentEventData,
   ToolStartEventData,
   ToolEndEventData,
+  AttachmentEventData,
   DoneEventData,
   ErrorEventData,
 } from './protocol.js';