Add AGENTS.md documentation for repository development workflow

[Copilot]

Adds comprehensive developer documentation in AGENTS.md covering repository structure, tooling, and workflows for both AI agents and human developers.

Contents

• Project structure - Directory layout and key file locations
• Development setup - PHP 8.1+, Composer, database installation (MySQL/SQLite)
• Code quality tools - PHPCS (PSR-12), Rector, syntax checking with configurations
• Testing - PHPUnit setup and execution patterns
• CI/CD - GitHub Actions matrix testing (PHP 8.1-8.4, MySQL/SQLite)
• Docker - Build and multi-platform deployment
• Plugin development - Standards and structure requirements
• Troubleshooting - Common issues and solutions
• Quick reference - Command snippets for frequent tasks

File is structured for rapid scanning with clear headings, code examples, and a command reference section at the end.

Original prompt

Write an AGENTS.md file describing how to work on this repository.

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