Add project instructions for FlightPHP core library

AGENTS.md

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+# FlightPHP/Core Project Instructions
+
+## Overview
+This is the main FlightPHP core library for building fast, simple, and extensible PHP web applications. It is dependency-free for core usage and supports PHP 7.4+.
+
+## Project Guidelines
+- PHP 7.4 must be supported. PHP 8 or greater also supported, but avoid PHP 8+ only features.
+- Keep the core library dependency-free (no polyfills or interface-only repositories).
+- All Flight projects are meant to be kept simple and fast. Performance is a priority.
+- Flight is extensible and when implementing new features, consider how they can be added as plugins or extensions rather than bloating the core library.
+- Any new features built into the core should be well-documented and tested.
+- Any new features should be added with a focus on simplicity and performance, avoiding unnecessary complexity.
+- This is not a Laravel, Yii, Code Igniter or Symfony clone. It is a simple, fast, and extensible framework that allows you to build applications quickly without the overhead of large frameworks.
+
+## Development & Testing
+- Run tests: `composer test` (uses phpunit/phpunit and spatie/phpunit-watcher)
+- Run test server: `composer test-server` or `composer test-server-v2`
+- Lint code & Check code style: `composer lint` (uses phpstan/phpstan, level 6)
+- Beautify code: `composer format` (uses squizlabs/php_codesniffer, PSR1)
+- Test coverage: `composer test-coverage`
+
+## Coding Standards
+- Follow PSR1 coding standards (enforced by PHPCS)
+- Use strict comparisons (`===`, `!==`)
+- PHPStan level 6 compliance
+- Focus on PHP 7.4 compatibility (avoid PHP 8+ only features)