feat: WS request proxy — Gateway proxies HTTP via WebSocket (#210 Phase 2)

- Add ws-protocol.ts with WsRequest/WsResponse types + parsers
- AgentSocket DO: proxy POST handler, pending request map, 30s timeout
- /api/agents/:agent/* routes through DO WS when connected, falls back to HTTP
- ws-client handles incoming WsRequest, fetches local serve, returns WsResponse
- startGatewayWsClient accepts localPort for request handling

Testing: #213

packages/workflow-gateway/package.json

@@ -3,6 +3,10 @@
   "version": "0.1.0",
   "private": true,
   "type": "module",
+  "exports": {
+    ".": "./src/index.ts",
+    "./ws-protocol": "./src/ws-protocol.ts"
+  },
   "scripts": {
     "dev": "wrangler dev",
     "deploy": "wrangler deploy"