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/openclaw.plugin.json

@@ -22,9 +22,9 @@
         "type": "string",
         "description": "Server identifier (auto-detected from hostname if not set)"
       },
-      "challengeUuid": { 
+      "apiKey": { 
         "type": "string",
-        "description": "Registration challenge UUID from Monitor"
+        "description": "API Key from HarborForge Monitor admin panel (optional but required for authentication)"
       },
       "reportIntervalSec": { 
         "type": "number", 
@@ -42,7 +42,6 @@
         "default": "info",
         "description": "Logging level"
       }
-    },
-    "required": ["challengeUuid"]
+    }
   }
 }