docs: add AGENTS.md agent guide and absolute README links

AGENTS.md

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+# AGENTS.md — OneSignal PHP SDK
+
+Integration guide for AI agents and LLMs using `onesignal/onesignal-php-api`, the OneSignal server SDK for PHP. Human-oriented docs are in the [README](./README.md).
+
+## What this SDK does
+
+Server-side access to the OneSignal REST API: send push / email / SMS, manage users, subscriptions, segments, templates and live activities, and administer apps & API keys.
+
+- Composer package: `onesignal/onesignal-php-api`
+- Repository: https://github.com/OneSignal/onesignal-php-api
+
+## Install
+
+```sh
+composer require onesignal/onesignal-php-api
+```
+
+## Authentication — two key types
+
+OneSignal uses two bearer credentials; each endpoint requires a specific one:
+
+- **REST API key** — used by almost every endpoint (notifications, users, subscriptions, segments, templates, live activities, custom events). Found in **App Settings → Keys & IDs**.
+- **Organization API key** — required *only* for organization-level endpoints: app management (list / create / get / update apps), API-key management (view / create / update / rotate / delete API keys), and copying a template to another app. Found in **Organization Settings**.
+
+Set both on the configuration; the SDK sends the correct credential per endpoint. Never hard-code keys — read them from environment variables or a secrets manager.
+
+```php
+use onesignal\client\api\DefaultApi;
+use onesignal\client\Configuration;
+use GuzzleHttp;
+
+$config = Configuration::getDefaultConfiguration()
+    ->setRestApiKeyToken(getenv('ONESIGNAL_REST_API_KEY'))
+    ->setOrganizationApiKeyToken(getenv('ONESIGNAL_ORGANIZATION_API_KEY'));
+
+$client = new DefaultApi(new GuzzleHttp\Client(), $config);
+```
+
+## Calling convention
+
+Methods take **positional arguments**. Build the model object and pass it directly — do **not** wrap arguments in a request/options object.
+
+```php
+$notification = new onesignal\client\Model\Notification();
+$notification->setAppId('YOUR_APP_ID');
+$contents = new onesignal\client\Model\LanguageStringMap();
+$contents->setEn('Hello from OneSignal!');
+$notification->setContents($contents);
+$notification->setIncludeAliases(['external_id' => ['YOUR_USER_EXTERNAL_ID']]);
+$notification->setTargetChannel('push');
+
+$response = $client->createNotification($notification);
+```
+
+## Idempotent sends & retries
+
+Set `idempotency_key` (a UUID) so a create-notification request can be safely retried — the server returns the original result instead of sending twice. The `NotificationHelpers::createNotificationWithRetry` helper handles this for you: it generates an `idempotency_key` when absent, retries `429` / `503` / transport errors with the **same** key (honoring `Retry-After`), and reports via `getWasReplayed()` whether the server answered from a previously completed request.
+
+```php
+use onesignal\client\helpers\NotificationHelpers;
+
+$result = NotificationHelpers::createNotificationWithRetry($client, $notification, 5, 1.0);
+echo 'id: ', $result->getResponse()->getId(), ' replayed: ', $result->getWasReplayed() ? 'true' : 'false', PHP_EOL;
+```
+
+> The notification-level `external_id` field is the **deprecated** idempotency mechanism — prefer `idempotency_key`. Don't confuse it with the `external_id` **alias label** (under `include_aliases`) used to target users.
+
+## Full API reference
+
+- [DefaultApi.md](https://github.com/OneSignal/onesignal-php-api/blob/main/docs/Api/DefaultApi.md) — every endpoint, parameter, and model, with runnable examples.
+- [OneSignal REST API reference](https://documentation.onesignal.com/reference)

CLAUDE.md

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+# CLAUDE.md
+
+AI agent and LLM integration guidance for this OneSignal server SDK lives in **[AGENTS.md](./AGENTS.md)** — authentication, calling conventions, idempotent retries, and the full API reference.
+
+See [AGENTS.md](./AGENTS.md).

README.md

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 # OneSignal
 
+> **Building with an AI agent or LLM?** See [AGENTS.md](https://github.com/OneSignal/onesignal-php-api/blob/main/AGENTS.md) for an agent-oriented integration guide — authentication, calling conventions, idempotent retries, and the full API reference.
+
 A powerful way to send personalized messages at scale and build effective customer engagement strategies. Learn more at onesignal.com
 
 For more information, please visit [https://onesignal.com](https://onesignal.com).