flutter_agent_kit_android library

The Android implementation of flutter_agent_kit.

Registration happens natively via the pluginClass declared in pubspec.yaml (FlutterAgentKitAndroidPlugin, in android/). This file exists so the package has a Dart entry point; the real bridge — wiring a FlutterAgentKitPlatform implementation backed by the Pigeon-generated host API — lands in Step 2 (Native bridge) of docs/v1-implementation-plan.md.