docs: add Copilot instructions for create-polyglot CLI

[kaifcoder]

No description provided.

[Copilot]

Pull Request Overview

This PR adds comprehensive Copilot instructions documentation to help GitHub Copilot understand the create-polyglot CLI project structure and conventions. The documentation provides detailed guidance on architecture, workflows, and development practices for the CLI tool that scaffolds polyglot monorepos.

• Adds complete project architecture documentation including core components and service selection logic
• Documents development workflows, testing patterns, and common pitfalls to avoid
• Provides quick reference guide for key files and external tool usage

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}