Update AGENTS.md to note Fantom runs as Android (#59185)

Summary:
Fantom tests run with `Platform.OS === 'android'` and have no iOS target, which the guide does not say, so an agent asked to test an iOS-only JavaScript branch writes a Fantom test that can never exercise it. Add that to the `yarn fantom` row, pointing at Jest for such branches.

Changelog: [Internal]

Pull Request resolved: #59185

Test Plan: Docs only. Rendered the table in a Markdown preview.

Reviewed By: Abbondanzo

Differential Revision: D126065356

Pulled By: fabriziocucci

fbshipit-source-id: eb06bebbd3f50221495ee25fe3d2ea4c51b36a38

Author: huntie

AGENTS.md

@@ -31,7 +31,7 @@ Architecture notes live in `__docs__` directories beside the code they describe,
 | Command | Purpose |
 | --- | --- |
 | `yarn test <path>` | Jest unit tests, found in `__tests__` directories |
-| `yarn fantom <path>` | [Fantom](private/react-native-fantom/__docs__/README.md) integration tests, named `*-itest.js` — builds a native tester on first run |
+| `yarn fantom <path>` | [Fantom](private/react-native-fantom/__docs__/README.md) integration tests, named `*-itest.js` — builds a native tester on first run, and runs as `Platform.OS === 'android'`, so an iOS-only branch needs Jest |
 | `yarn lint` | ESLint (`--max-warnings 0`) |
 | `yarn flow-check` | Flow |
 | `yarn format` | Format JavaScript/TypeScript, C/C++/Objective-C/protobuf, Kotlin, Java, Python, and Swift sources |