Add project documentation

docs/communications.md

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+# Communications
+
+## Architecture
+
+Pico Dashboard
+↓
+RS-485
+↓
+ESP32 Controller
+
+Backup:
+WiFi
+
+## Protocol
+
+Message Types:
+
+- status_request
+- status_response
+- set_relay
+- relay_response
+- enable_wifi
+- wifi_response
+- toggle_ignition
+- ignition_response
+- toggle_sensor_fault
+- sensor_fault_response
+
+## Failure Handling
+
+If RS-485 is lost:
+- Dashboard displays warning
+- Last known values remain visible
+- Relay commands fail safely
+
+## Future Features
+
+- MQTT
+- Home Assistant
+- OTA Updates
+- Remote Diagnostics