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

[LTechnologies0]

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

What was set up

• OpenJDK 17 (used by this app) and 21 installed; Android SDK at $HOME/android-sdk.
• local.properties (gitignored) generated with sdk.dir.

Verified in this environment

• ✅ ./gradlew :app:assembleDebug -Psecurefilemanager.devAbi=arm64-v8a --no-daemon → app/build/outputs/apk/debug/secure-file-manager-debug.apk (valid launchable app ltechnologies.onionphone.securefilemanager.debug/.activities.MainActivity).
• ✅ ./gradlew test --no-daemon
• ✅ ./gradlew :app:lintDebug --no-daemon runs and generates reports (aborts on pre-existing findings by design).

Notes

• Uses JDK 17 (JAVA_HOME=/usr/lib/jvm/java-17-openjdk-amd64).
• 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.