docs: add AGENTS.md with Cursor Cloud dev environment notes

[LTechnologies0]

Adds AGENTS.md documenting how to build, test, lint, and run SecureMessenger in the Cursor Cloud environment.

What was set up

• OpenJDK 21 (used by this app) and 17 installed; Android SDK at $HOME/android-sdk.
• local.properties (gitignored) generated with sdk.dir + telegram stub values.
• Prebuilt TDLib AAR fetched via ./scripts/fetch-tdlib-prebuilt.sh into protocol/telegram/libs/ (gitignored).

Verified in this environment

• ✅ ./gradlew :app:assembleDebug -Psecuremessenger.devAbi=arm64-v8a --no-daemon → app/build/outputs/apk/debug/app-debug.apk (valid launchable app ltechnologies.onionphone.securemessenger/.ui.MainActivity).
• ✅ ./gradlew test --no-daemon

Notes

• Uses JDK 21 (JAVA_HOME=/usr/lib/jvm/java-21-openjdk-amd64).
• The prebuilt TDLib AAR is a build prerequisite; abiFilters links its .sos so no NDK/CMake compilation is needed.
• Empty telegram.api.id/telegram.api.hash are fine for building/testing; real Telegram connectivity needs valid values.
• Headless VM has no /dev/kvm, so a GUI emulator cannot run.

Only AGENTS.md is added; no source changes.

  

[github-actions]

Thanks for your pull request! CI runs tests, debug APK build, and CodeQL.
Please add KDoc for new public APIs and ensure no secrets are committed.