✨ Add Copilot instructions for Infection mutation testing framework

[Copilot]

This PR adds comprehensive Copilot instructions to help GitHub Copilot better understand and assist with the Infection mutation testing framework.

The .github/copilot-instructions.md file includes:

• Repository overview: Clear explanation of mutation testing concepts and Infection's purpose
• Architecture guide: Detailed breakdown of the src/ directory structure and key components
• Technology stack: PHP 8.2+, Symfony components, and other dependencies
• Development workflow: Setup instructions, quality assurance commands (make test, make cs, etc.)
• Coding standards: PSR-12 compliance, strict typing, immutable objects, and type safety
• Testing philosophy: Unit, integration, and end-to-end testing approaches
• Common patterns: Examples for mutator implementation, value objects, and configuration classes
• Performance considerations: Memory efficiency and parallel execution guidance
• Contribution guidelines: Best practices for maintaining this production-ready tool

This will enable Copilot to provide more accurate and contextually relevant assistance when working with Infection's codebase, understanding its unique requirements as a mutation testing framework that processes large codebases and runs extensive test executions.

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[Copilot]

@maks-rafalko 👋 This repository doesn't have Copilot instructions. With Copilot instructions, I can understand the repository better, work faster and produce higher quality PRs.

I can generate a .github/copilot-instructions.md file for you automatically. Click here to open a pre-filled issue and assign it to me. I'll write the instructions, and then tag you for review.

[maks-rafalko]

@copilot retry, your error is wierd

[Copilot]

@copilot retry, your error is wierd

I've successfully created the .github/copilot-instructions.md file for the Infection mutation testing framework. The instructions include comprehensive information about the repository architecture, technology stack, development workflow, coding standards, and contribution guidelines specific to mutation testing. This will help me better understand the codebase and provide more accurate assistance. Commit f2547b0

[maks-rafalko]

let's see if it helps