Configure Copilot agent setup for PHP 8.4 and authenticated Composer installs

[Copilot]

Copilot sessions were running on PHP 8.3, which is incompatible with this codebase and causes analysis/test failures. This PR adds a dedicated Copilot setup workflow to pin PHP 8.4 and use authenticated Composer installs in Copilot runs.

• Copilot environment bootstrap (new workflow)

  • Added .github/workflows/copilot-setup-steps.yml with required copilot-setup-steps job.
  • Configures the agent runtime to PHP 8.4 via shivammathur/setup-php@v2.
  • Uses authenticated setup token with fallback:
    • secrets.GH_PAT_TOKEN || github.token.

• Dependency install configuration for Copilot sessions

  • Uses composer install --no-interaction --no-progress --prefer-dist in setup steps.

• Agent guidance update

  • Updated CLAUDE.md to explicitly state PHP 8.4+ requirement for agent-run dependency/test validation in this repository context.

- name: Setup PHP 8.4
  uses: shivammathur/setup-php@v2
  with:
    php-version: '8.4'
    github-token: ${{ secrets.GH_PAT_TOKEN || github.token }}
    tools: composer:v2
    coverage: none
    ini-values: memory_limit=-1

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