feat: implement MQTT client for edge reporting

## Description
Add MQTT client to report sensor data to NanoClaw (L2) or PicoClaw (L1) gateway.

## Requirements
- [ ] MQTT connection with configurable broker URL
- [ ] Publish sensor readings on configurable interval
- [ ] Auto-reconnect on connection loss
- [ ] Topic format: clawland/{node_id}/sensors/{sensor_type}

## Skills
C/C++, MQTT, ESP32