Add GitHub Copilot instructions

[Copilot]

Configures custom instructions for GitHub Copilot coding agent per best practices.

Changes

• Created .github/copilot-instructions.md with repository-specific guidance:
  • Docker Compose command patterns for setup, testing, and builds
  • Code standards: PSR-12, strict typing, service layer architecture
  • Testing pyramid (unit > integration > functional) and coverage expectations
  • Security requirements and PR checklist
  • References to existing documentation (AGENTS.md, docs/)

The instructions provide Copilot with the context needed to work within this Symfony 7.3 + Docker environment, emphasizing minimal changes and established patterns.

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 #229

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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.