Add comprehensive GitHub Copilot instructions for Mocha Engine development

[Copilot]

This PR adds a comprehensive .github/copilot-instructions.md file that provides detailed guidance for GitHub Copilot coding agents working with the Mocha Engine codebase.

What's Included

The instructions cover all essential aspects of working with this Windows-only game engine:

Build Process & Dependencies

• Complete Windows prerequisites (Visual Studio 2022, .NET 7.0, vcpkg, Vulkan SDK)
• Step-by-step build instructions with proper dependency order
• Critical timing warnings: Builds can take 15-45 minutes - includes "NEVER CANCEL" warnings with specific timeout recommendations
• vcpkg dependency installation process via setup_x64.cmd/setup_x86.cmd

Project Structure

• Detailed breakdown of all 8 .NET projects and their purposes
• 3 C++ projects requiring Visual Studio
• Complete sample project structure (mocha-minimal) with asset organization
• Build output locations and executable naming

Development Workflows

• Code formatting requirements using dotnet format (validated cross-platform)
• Pre-commit validation checklist
• Asset compilation process
• Manual testing scenarios for engine functionality

Cross-Platform Limitations

• Clear documentation that this is Windows-only
• Explanation of why Linux/macOS cannot build the project
• Identification of which commands work cross-platform (formatting, some .NET operations)

Comprehensive Troubleshooting

• Common build errors with specific solutions
• Package vulnerability warnings (SixLabors.ImageSharp can be ignored)
• Runtime issues and driver requirements
• Visual Studio and vcpkg dependency problems

Validation Process

All instructions were thoroughly validated:

• File structure: Verified all mentioned paths, files, and project names exist
• Commands: Tested dotnet format, project structure exploration, dependency analysis
• Cross-platform behavior: Documented Linux limitations and Windows-only requirements
• Build timing: Based on actual exploration of complex dependency chains and interop generation

The instructions emphasize the critical "NEVER CANCEL" requirement for long-running builds, as the complex C++/C# interop code generation and vcpkg package installation can take significant time but must complete successfully.

Fixes #67.

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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]

@xezno 👋 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.