✨ Set up comprehensive Copilot instructions for Verifactu PHP library

[Copilot]

This PR adds comprehensive GitHub Copilot instructions to help developers understand the codebase patterns and contribute effectively to the Verifactu PHP library.

What's Added

Created .copilot-instructions.md with detailed guidance covering:

Library Context & Purpose

• Spanish AEAT Verifactu digital invoicing system integration
• Framework-agnostic design with PHP 8.1+ compatibility
• PSR compliance and DRY principle adherence
• Composer-based installation and dependency management

Architecture & Design Patterns

• Facade pattern for the main Verifactu class API
• Abstract Model pattern with validation framework
• Service layer pattern for business logic separation
• Registry pattern for error code management

Development Standards

• PHP 8.1+ features usage (enums, match expressions, typed properties)
• PSR-4 autoloading and PSR-12 coding style compliance
• Naming conventions and documentation standards
• Model validation patterns and XML handling

Testing & Quality

• PHPUnit 10+ testing patterns
• Model validation testing approaches
• Service mocking and exception testing
• Code quality and contribution guidelines

Domain-Specific Knowledge

• Spanish tax system context and AEAT regulatory compliance
• Certificate management and XML security considerations
• Error handling with official AEAT error code translations
• Environment management (production vs sandbox)

The instructions provide GitHub Copilot with comprehensive context about the library's purpose, architecture patterns, coding conventions, and domain-specific requirements. This will enable more accurate code suggestions and help new contributors understand the codebase structure and Spanish invoicing system requirements.

Fixes #16.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

• https://api.github.com/repos/DASPRiD/Enum/zipball/8dfd07c6d2cf31c8da90c53b83c026c7696dda90
  • Triggering command: /usr/bin/php8.3 -n -c /tmp/iZOjhX /usr/bin/composer install (http block)
  • Triggering command: /usr/bin/php8.3 -n -c /tmp/rZ7O6h /usr/bin/composer install --no-dev (http block)
• https://api.github.com/repos/sebastianbergmann/object-enumerator/zipball/d8bd7ec3598ee22d95317e91ef01bf6b445ffb08
  • Triggering command: /usr/bin/php8.3 -n -c /tmp/iZOjhX /usr/bin/composer install (http block)
• https://api.github.com/repos/sebastianbergmann/type/zipball/3d153476a10dd4400cfb3c002349a4fdde7f51a8
  • Triggering command: /usr/bin/php8.3 -n -c /tmp/iZOjhX /usr/bin/composer install (http block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.