feat: fix API Key authentication and payload alignment

- Update openclaw.plugin.json: replace challengeUuid with apiKey (optional)
- Fix tsconfig: use CommonJS module to avoid import.meta.url issues
- Fix plugin/index.ts: remove ESM-specific code, use __dirname
- Fix telemetry.mjs:
  - Add loadavg to os imports, remove require() call
  - Replace challengeUuid with apiKey in config
  - Update endpoint to heartbeat-v2
  - Add X-API-Key header when apiKey is configured
  - Fix payload field names: agents, load_avg (array), uptime_seconds
  - Change missing apiKey from error to warning

plugin/tsconfig.json

@@ -1,8 +1,8 @@
 {
   "compilerOptions": {
     "target": "ES2022",
-    "module": "NodeNext",
-    "moduleResolution": "NodeNext",
+    "module": "CommonJS",
+    "moduleResolution": "node",
     "esModuleInterop": true,
     "strict": true,
     "skipLibCheck": true,