Add Copilot instructions for HAVIT Blazor project template

[Copilot]

This PR adds comprehensive Copilot instructions to help developers work effectively with this project template and projects derived from it.

What's Added

A new .github/copilot-instructions.md file that provides:

• Project Architecture Overview: Clear explanation of the clean architecture pattern with all project layers (Web.Server, Web.Client, Model, Entity, DataLayer, Services, Facades, etc.)

• Technology Stack Documentation: Complete coverage of .NET 9, Blazor Server/WebAssembly, Entity Framework Core 9, HAVIT Framework, Hangfire, and testing frameworks

• Coding Guidelines: Naming conventions, Entity Framework patterns specific to HAVIT's repository pattern, business logic organization, and error handling best practices

• Template Usage Guidelines: Step-by-step guidance for initial setup using SetupSolution.ps1, code generation workflows, and configuration management

• Best Practices: Performance optimization, security considerations, maintainability patterns, and migration strategies

• Code Examples: Practical examples showing repository patterns, service registration, dependency injection, and Blazor component patterns

• Troubleshooting: Common issues and solutions for build errors, migrations, DI registration, and component lifecycle management

Benefits

This enables Copilot to provide more accurate and contextually appropriate suggestions when working with:

• HAVIT Framework patterns and conventions
• Entity Framework code generation workflows
• Blazor Server/WebAssembly development
• Clean architecture implementation
• Template customization and project setup

The instructions are specifically tailored for this project template's architecture and will help maintain consistency across all projects derived from this template.

Fixes #9.

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