Add GitHub Copilot instructions for repository

[Copilot]

Configures GitHub Copilot coding agent with repository-specific context per best practices.

Changes

• Added .github/copilot-instructions.md with:
  • Magento 2 module context (PHP 8.3+, requires 2.4.7+)
  • Coding standards: Magento Coding Standards, PSR-4, strict typing with declare(strict_types=1), constructor property promotion with readonly
  • Code formatting: 4-space indentation, 80-char lines, PascalCase classes, camelCase methods
  • Build validation: PHPCS with Magento2 standard
  • Git conventions: #<issue> - <message> commit format
  • Project structure: Command patterns, service layer, theme builder interface implementations
  • Domain guidance: Theme builders in Service/ThemeBuilder/, Magento DI patterns, CLI command shortcodes

This enables Copilot to generate code consistent with project conventions and understand the Magento 2 module architecture.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>✨ Set up Copilot instructions</issue_title>
<issue_description>Configure instructions for this repository as documented in Best practices for Copilot coding agent in your repository.

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes ✨ Set up Copilot instructions #47

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