Add Home Assistant custom integration and ESP availability tracking

- Add custom HA integration (config flow, switch, water_for service)
- Add MQTT Last Will & Testament for offline detection
- Publish 'online' on successful MQTT connect
- Update README with integration docs

irrigation/README.md

@@ -1,15 +1,41 @@
 # Irrigation with an ESP8266
 Tested with D1 mini. Connect a pump with a relais. Relais activation on D4.
 
-## Homeassistant integration
-payload_on can also contain the duration of the pump in ms.
-Example:
-`
-switch 5:
-- platform: mqtt
-  command_topic: greenhousino/pump
-  state_topic: greenhousino/pumpstate
-  payload_on: "on"
-  payload_off: "off"
-  name: "Pumpe"
-`
+## Home Assistant Integration
+
+### Custom Integration (recommended)
+
+A full custom integration with config flow, device registry, and a `water_for` service is included in the [`homeassistant/`](homeassistant/) directory.
+
+See [homeassistant/README.md](homeassistant/README.md) for installation and usage.
+
+### YAML-only (built-in MQTT platform)
+
+`payload_on` can also contain the duration of the pump in ms.
+
+```yaml
+switch:
+  - platform: mqtt
+    name: "Irrigation Pump"
+    command_topic: greenhousino/pump
+    state_topic: greenhousino/pumpstate
+    payload_on: "on"
+    payload_off: "off"
+    state_on: "on"
+    state_off: "off"
+    availability_topic: greenhousino/pump/status
+    payload_available: "online"
+    payload_not_available: "offline"
+    qos: 1
+```
+
+For duration-based watering via YAML, use an automation:
+```yaml
+automation:
+  - alias: "Water for 5 minutes"
+    action:
+      - service: mqtt.publish
+        data:
+          topic: greenhousino/pump
+          payload: "300000"  # 300000 ms = 5 minutes
+```