chore(ai): add smoke test script (agent)

This commit ensures the existence and correctness of `scripts/agent/smoke-test.mjs`, a Playwright-based regression test suite.

The script:
- Starts a local web server (npx serve) and a local relay (simple-relay.mjs).
- Launches a headless browser to interact with the application.
- Verifies key flows:
  - Ephemeral login.
  - Video publishing (Kind 30078).
  - Direct Message (DM) sending and decryption (Kind 4/1059).
- Generates execution logs and summaries in `artifacts/`.

Also ensures `scripts/agent/simple-relay.mjs` is present as a dependency.

scripts/agent/smoke-test.mjs

@@ -1,3 +1,7 @@
+/**
+ * Smoke test script for verifying key flows (login, video publish, DM).
+ * Run with: node scripts/agent/smoke-test.mjs
+ */
 import { chromium } from 'playwright';
 import { spawn } from 'child_process';
 import { startRelay } from './simple-relay.mjs';