docs: add cursor cloud setup instructions

[ben-kaufman]

Documents the Cursor Cloud development environment setup for this Android/Kotlin app so future cloud agents can build, test, and run it.

Description

• Adds a ## Cursor Cloud specific instructions section to AGENTS.md capturing non-obvious environment caveats:
  • Build tooling baked into the VM snapshot: JDK 17, Android SDK (platform 36, build-tools 36.0.0, platform-tools), and just. Gradle picks JDK 17 via ~/.gradle/gradle.properties and the SDK via local.properties so ./gradlew works in non-interactive shells too.
  • The com.synonym:* / paykit native libraries resolve from private GitHub Packages Maven repos and REQUIRE GITHUB_ACTOR + GITHUB_TOKEN (PAT with read:packages). Without them every classpath-resolving Gradle task fails with Username must not be null!.
  • Unit tests run on the JVM via Robolectric (no emulator); just test android/UI/E2E need an emulator.
  • NDK only needed for release builds; google-services.json placeholder is checked in for dev/testnet debug.

No source code changes. local.properties is gitignored and not committed.

Preview

N/A — documentation-only change.

QA Notes

• ./gradlew projects (config-only, no credentials) configures successfully using JDK 17 and the installed SDK.
• Full build/test/run is blocked until the GITHUB_ACTOR / GITHUB_TOKEN secrets are added, since the native libraries cannot otherwise be resolved.