Create copilot-instructions.md

[Masterain98]

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[Copilot]

Pull Request Overview

This PR creates a comprehensive Copilot instructions file to provide guidance for AI assistants working with the Snap.Hutao.Server codebase. The file aims to reduce the need for agents to search through the codebase by providing essential context about the project structure, build processes, and development workflows.

• Creates a detailed guidance document covering project overview, build instructions, and codebase architecture
• Provides specific commands and setup procedures for development and testing
• Documents key patterns and conventions used throughout the codebase

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