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/homeassistant/README.md

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+# Greenhousino Irrigation — Home Assistant Integration
+
+Custom Home Assistant integration for the Greenhousino ESP8266/ESP32 irrigation controller.
+
+## Features
+
+- **Pump switch** — Turn the irrigation pump on/off from Home Assistant
+- **State tracking** — Real-time pump state via MQTT
+- **Availability** — Shows online/offline status
+- **`water_for` service** — Water for a specific duration (1–3600 seconds)
+- **Config flow** — Set up via Home Assistant UI (Settings → Devices & Services → Add Integration)
+- **Device registry** — Appears as a proper device with manufacturer/model info
+
+## Installation
+
+### Option 1: HACS (recommended)
+
+1. Install [HACS](https://hacs.xyz/) in Home Assistant
+2. Add this repository as a custom repository
+3. Search for "Greenhousino Irrigation" and install
+4. Restart Home Assistant
+
+### Option 2: Manual installation
+
+1. Copy the `greenhousino` folder to your Home Assistant config directory:
+
+```bash
+cp -r greenhousino /config/custom_components/
+```
+
+2. Restart Home Assistant
+
+3. Go to **Settings → Devices & Services → Add Integration** and search for "Greenhousino Irrigation"
+
+## Configuration
+
+The config flow will guide you through two steps:
+
+### Step 1: Host
+- **Host IP address** — The IP of your ESP device (e.g., `192.168.178.50`). This is used for the web UI link in device info.
+
+### Step 2: MQTT Topics
+- **Command topic** — MQTT topic to send commands (default: `greenhousino/pump`)
+- **State topic** — MQTT topic for state updates (default: `greenhousino/pumpstate`)
+
+## Usage
+
+### Switch entity
+
+The integration creates a switch entity (`switch.greenhousino_irrigation_pump`) that you can:
+- Toggle on/off from the Home Assistant UI
+- Use in automations
+- Add to dashboards
+
+### `water_for` service
+
+Water for a specific duration:
+
+```yaml
+service: greenhousino.water_for
+data:
+  duration: 60  # seconds
+```
+
+Or from an automation:
+```yaml
+automation:
+  - alias: "Water garden every morning"
+    trigger:
+      - platform: time
+        at: "06:00:00"
+    action:
+      - service: greenhousino.water_for
+        data:
+          duration: 300  # 5 minutes
+```
+
+## MQTT Topics
+
+| Direction | Topic | Payload | Description |
+|-----------|-------|---------|-------------|
+| HA → ESP | `greenhousino/pump` | `on` | Turn pump on (default 30s) |
+| HA → ESP | `greenhousino/pump` | `off` | Turn pump off |
+| HA → ESP | `greenhousino/pump` | `5000` | Turn pump on for 5000ms |
+| ESP → HA | `greenhousino/pumpstate` | `on` | Pump is active |
+| ESP → HA | `greenhousino/pumpstate` | `off` | Pump is off |
+| ESP → HA | `greenhousino/pump/status` | `offline` | Device disconnected |
+
+## Requirements
+
+- Home Assistant 2024.x or later
+- MQTT integration configured and connected to the same broker as your ESP device
+
+## Troubleshooting
+
+### Switch shows "unavailable"
+- Make sure the MQTT integration is connected
+- Verify the ESP device is publishing to the state topic
+- Check that topics match between the ESP firmware and HA config
+
+### Service call doesn't work
+- Ensure MQTT integration is set up
+- Check HA logs: `Logger → greenhousino`
+- Verify the ESP is subscribed to the command topic
+
+## YAML-only alternative (no custom component)
+
+If you don't want a custom integration, you can use the built-in MQTT platform:
+
+```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
+```