Implement browser session inactivity timeout and cleanup

- Updated `.env.example` to include `BROWSER_INACTIVITY_TIMEOUT` for auto-cleanup of inactive sessions.
- Enhanced `cli.py` to load the new inactivity timeout configuration into environment variables.
- Added background thread functionality in `browser_tool.py` to periodically clean up inactive browser sessions based on the configured timeout.
- Improved session management by tracking last activity timestamps and ensuring cleanup occurs when sessions exceed inactivity limits.

package-lock.json

@@ -7,9 +7,13 @@
     "": {
       "name": "hermes-agent",
       "version": "1.0.0",
-      "license": "ISC",
+      "hasInstallScript": true,
+      "license": "MIT",
       "dependencies": {
         "agent-browser": "^0.7.6"
+      },
+      "engines": {
+        "node": ">=18.0.0"
       }
     },
     "node_modules/agent-browser": {