docs: add AGENTS.md with Cursor Cloud setup instructions

[yannelli]

Summary

Sets up the Cursor Cloud development environment for the yannelli/attempt Laravel library and documents durable, non-obvious setup/run guidance for future cloud agents.

This is a Composer library (no app/server/DB). Development readiness = installing PHP 8.4 + Composer, installing dependencies, and running the Pest/Testbench test suite.

Changes

• Add AGENTS.md with a ## Cursor Cloud specific instructions section covering: library-not-app clarification, standard lint/test commands, and non-obvious caveats (missing composer analyse/format:test scripts, PHPStan not in require-dev, and how to boot a Laravel container for ad-hoc standalone scripts via Testbench's skeleton base path).

Environment

• Update script: composer install --no-interaction (idempotent; no composer.lock so it resolves fresh).
• PHP 8.4 + required extensions and Composer are installed at the VM/system level.

Verification

• Lint: vendor/bin/pint --test → passed.
• Tests: vendor/bin/pest --ci → 161 passed (332 assertions).
• Hello-world: booted a minimal Laravel container via Orchestra Testbench and exercised core retry (succeeded on 3rd attempt) + fallback (returned default) behavior.