feat: implement MQTT client for edge reporting (#3)

[addidea]

Description

Implements MQTT client for MicroClaw to report sensor data to NanoClaw/PicoClaw gateways.

Changes

• ✅ MQTT connection with configurable broker URL ()
• ✅ Publish sensor readings on configurable interval (default 60 seconds)
• ✅ Auto-reconnect on connection loss with 5-second retry interval
• ✅ Topic format: clawland/{node_id}/sensors/{sensor_type}
• ✅ Integrated with existing DHT22 sensor driver
• ✅ Status topic for online/offline tracking
• ✅ Comprehensive README with MQTT setup, topics, configuration, and troubleshooting

MQTT Topics

• Temperature: clawland/node_001/sensors/temperature
• Humidity: clawland/node_001/sensors/humidity
• Status: clawland/node_001/status

Configuration

All MQTT parameters are configurable in src/main.cpp:

const char* MQTT_BROKER = "mqtt.clawland.local";
const char* NODE_ID = "node_001";
const int PUBLISH_INTERVAL = 60000;  // 60 seconds

Auto-Reconnect

• WiFi reconnection on connection loss
• MQTT reconnection with 5-second intervals
• Graceful degradation if broker is unavailable

Testing

Tested with:

• Broker: Mosquitto MQTT broker
• Scenario 1: Normal operation (60-second publish cycle)
• Scenario 2: Broker disconnect/reconnect (auto-recovery verified)
• Scenario 3: WiFi disconnect/reconnect (auto-recovery verified)

Dependencies

Uses existing PubSubClient library already in platformio.ini — no new dependencies.

Closes #3