Add AGENTS.md for Cursor Cloud development environment

[abdulraqeeb33]

Description

One Line Summary

Adds AGENTS.md with Cursor Cloud-specific development environment instructions for the OneSignal Android SDK.

Details

Motivation

Enables Cursor Cloud agents to quickly set up and work with this Android SDK codebase by documenting prerequisites, key commands, and gotchas discovered during environment setup.

Scope

Only adds a new AGENTS.md file. No existing code is modified.

Testing

Environment validation performed:

• Build: ./gradlew assembleRelease — all 358 tasks successful (SDK modules + demo app)
• Debug build: ./gradlew :app:assembleGmsDebug — APK produced at app-gms-debug.apk (19.5 MB)
• Lint (formatting): ./gradlew spotlessCheck — passed (61 tasks)
• Lint (static analysis): ./gradlew detekt — passed with pre-existing warnings
• Unit tests: ./gradlew testDebugUnitTest — 982 tests run, 978 passed, 4 pre-existing failures

Build log evidence:
gradle_build.log
gradle_test.log

Checklist

Overview

• I have filled out all REQUIRED sections above
• PR does one thing — adds development environment documentation
• No Public API changes

Testing

• All automated tests pass (4 pre-existing failures unrelated to this change)
• Verified build, lint, and test commands work correctly

Final pass

• Code is as readable as possible
• I have reviewed this PR myself

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