Add copilot-setup-steps.yml for GitHub Copilot environment customization

[Copilot]

This PR adds a .github/copilot-setup-steps.yml file to customize GitHub Copilot's coding agent environment, following the best practices outlined in GitHub's documentation.

What this enables

The copilot-setup-steps.yml file ensures that GitHub Copilot has access to the same development environment as our CI pipeline, including:

• PHP 8.4+ - Critical requirement for this project due to property hooks syntax
• Composer dependency management - All development tools and libraries
• Development toolchain - php-cs-fixer, PHPStan, PHPUnit, composer-dependency-analyser
• Performance optimization - Composer package caching using the same strategy as CI

Implementation details

The setup file mirrors our CI workflow (.github/workflows/ci.yml) but focuses specifically on environment preparation rather than validation:

steps:
  - name: Setup PHP 8.4
    uses: shivammathur/setup-php@v2
    with:
      php-version: '8.4'
      coverage: none
      tools: composer

  - name: Cache Composer packages
    uses: actions/cache@v4
    # ... caching configuration

  - name: Install dependencies
    run: composer install --prefer-dist --no-progress

  - name: Verify development tools are available
    run: |
      vendor/bin/composer-dependency-analyser --version
      vendor/bin/php-cs-fixer --version  
      vendor/bin/phpstan --version
      vendor/bin/phpunit --version
      composer normalize --version

Benefits

With this setup, GitHub Copilot will be able to:

• Better understand the project's PHP 8.4+ syntax and features
• Access the same development tools used in CI for code analysis
• Provide more accurate suggestions based on the project's actual dependencies
• Understand the coding standards enforced by php-cs-fixer and PHPStan

This follows the project's philosophy of maintaining consistency between development, CI, and now AI-assisted development environments.

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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.