feat: add load testing script

- Implement `scripts/agent/load-test.mjs` for stress testing local relays.
- Simulate N clients publishing Video Post and View Events.
- Measure latency (Build, Sign, RTT), throughput, and system resources.
- Update `scripts/agent/simple-relay.mjs` to support configurable `PORT`.
- Include `artifacts/load-report-20260122.json` from a 1000-client run.

scripts/agent/simple-relay.mjs

@@ -1,6 +1,6 @@
 import { WebSocketServer } from 'ws';
 
-const PORT = 8008;
+const PORT = process.env.PORT || 8008;
 const wss = new WebSocketServer({ port: PORT });
 
 // Simple in-memory store for REQ matching