Migrate unit tests to PlatformIO Unity

Replace the legacy tests/ tree (AUnit, EpoxyDuino, Make, AUniter) with
PlatformIO Unity suites under test/, shared helpers, and platformio.ini.
Remove AUnitHelpers and drop the AUnit include from ArduinoHA test builds.
Refresh docs to Markdown-only and document the test workflow in CHANGELOG.

docs/getting-started.md

@@ -0,0 +1,92 @@
+# Getting started
+
+## Prerequisites
+
+ArduinoHA talks to Home Assistant over **MQTT** (TCP). You need an MQTT broker reachable from your board. On Home Assistant OS, the [Mosquitto add-on](https://github.com/home-assistant/addons/blob/master/mosquitto/DOCS.md) is a common choice.
+
+## Install the library
+
+**Arduino IDE:** Library Manager → search for `home-assistant-integration` → Install.
+
+**PlatformIO:** add the dependency in `platformio.ini` (see the project [`library.json`](../library.json) for PubSubClient):
+
+```ini
+lib_deps =
+    https://github.com/alexhopeoconnor/arduino-home-assistant.git
+```
+
+You also need a network `Client` (Ethernet or Wi‑Fi) compatible with the Arduino networking API.
+
+## Minimal layout
+
+1. Create **`HADevice`** and **`HAMqtt`** once (global or inside a long-lived object).
+2. Call **`HAMqtt::begin(...)`** once, at the **end** of `setup()` — it only stores broker settings; the actual connection runs during **`HAMqtt::loop()`**.
+3. Call **`mqtt.loop()`** regularly in `loop()` (not necessarily every iteration).
+4. Construct **entity classes** (sensors, switches, …) **after** `HAMqtt`, and register them before `begin()` where the API requires it.
+
+### Ethernet (example)
+
+```cpp
+#include <Ethernet.h>
+#include <ArduinoHA.h>
+
+byte mac[] = {0x00, 0x10, 0xFA, 0x6E, 0x38, 0x4A};
+EthernetClient client;
+HADevice device(mac, sizeof(mac));
+HAMqtt mqtt(client, device);
+
+void setup() {
+    Ethernet.begin(mac);
+    mqtt.begin("192.168.1.50", "mqtt_user", "mqtt_password");
+}
+
+void loop() {
+    Ethernet.maintain();
+    mqtt.loop();
+}
+```
+
+### ESP8266 / ESP32 (example)
+
+```cpp
+#include <ESP8266WiFi.h>
+#include <ArduinoHA.h>
+
+WiFiClient client;
+HADevice device;
+HAMqtt mqtt(client, device);
+
+void setup() {
+    byte mac[WL_MAC_ADDR_LENGTH];
+    WiFi.macAddress(mac);
+    device.setUniqueId(mac, sizeof(mac));
+
+    WiFi.begin("SSID", "password");
+    while (WiFi.status() != WL_CONNECTED) {
+        delay(500);
+    }
+
+    mqtt.begin("192.168.1.50", "mqtt_user", "mqtt_password");
+}
+
+void loop() {
+    mqtt.loop();
+}
+```
+
+## `begin()` variants
+
+All are valid; pick one. Hostnames work instead of IP addresses.
+
+- `mqtt.begin("192.168.1.50")` — anonymous, port 1883  
+- `mqtt.begin("192.168.1.50", 8888)` — anonymous, custom port  
+- `mqtt.begin("192.168.1.50", "user", "pass")` — credentials, port 1883  
+- `mqtt.begin("192.168.1.50", 8888, "user", "pass")` — credentials + custom port  
+
+## Security note
+
+Credentials go over **plain TCP** unless you use a TLS-capable stack and broker setup. On a trusted LAN this is often acceptable; treat untrusted networks accordingly.
+
+## Examples
+
+See [`examples/`](../examples/) — start with `nodemcu` / `nano33iot` or an entity example matching what you need.