docs: Add AI coding agent instructions

[fbraz3]

Add AI coding agent instructions

This PR adds .github/copilot-instructions.md to streamline the development process by providing AI assistants and new contributors with immediate access to project-critical knowledge.

Development workflow improvements:

• Faster onboarding: New developers can understand build requirements and coding standards immediately
• Consistent code quality: AI assistants will follow TheSuperHackers comment format and project conventions
• Reduced review cycles: Contributors understand VC6 compatibility requirements and testing procedures upfront
• Better AI assistance: Copilot/Cursor get context about legacy C++98 patterns, GameLogic/GameClient separation
• Avoid common mistakes: Documents critical aspects like replay compatibility testing and Zero Hour precedence

Key knowledge documented:

• CMake presets and VC6 build requirements for retail compatibility
• Mandatory TheSuperHackers comment format and conventional commits
• Replay testing procedures that prevent breaking retail compatibility
• Dual game architecture and engine separation patterns

Impact on development:

Instead of learning project specifics through trial/error or lengthy discussions, contributors can immediately understand what makes this project unique and work productively from day one.

[M1SAM18]

Having someone to montior the AI would also be necessary to keep up to date with the work that is already done, in order to avoid reopening issues that have already been solved or is being worked on or its already located in another issue elsewhere in issue section.

This must be done in order to avoid waste of time and unnecessary increase of work since whoever has not followed the project and are new could create confusion and slow down the workflow.

That being said If the github AI can keep up to date as to what is currently being worked on and what has been finished in a wiki format that would be great and much appreciated.

[xezon]

I am not using AI agents so I dont know how useful this is but looks like it will not hurt.