Add comprehensive GitHub Copilot instructions for C# template repository

[Copilot]

This PR adds a comprehensive .github/copilot-instructions.md file that provides GitHub Copilot coding agents with detailed instructions for working effectively in this C# template repository.

Key Features

Prerequisites and Environment Setup

• Exact commands for installing .NET 9.0 SDK (version 9.0.304) with timing expectations
• Path configuration and verification steps
• Measured installation time: 30-60 seconds with appropriate timeout warnings

Development Workflow

• Complete project creation workflows for console apps, class libraries, web APIs, and test projects
• Build, test, and run commands with measured timing (~5 seconds each)
• Code formatting and linting using dotnet format with .editorconfig rules
• Package management and project reference commands

Validation Scenarios

• Step-by-step validation process that can be copy-pasted to verify the template works
• Manual testing scenarios including creating, building, running, and testing projects
• Expected outputs and timing for each step

Comprehensive Command Reference

• Solutions and project management
• Publishing and deployment options
• Common NuGet packages and their installation commands
• Web development specific guidance including hot reload (dotnet watch)
• Performance and diagnostic commands

Configuration Documentation

• Explanation of key files: global.json, Directory.Build.props, .editorconfig
• Project structure and dev container configuration
• Troubleshooting guidance for common issues

Validation

All instructions have been thoroughly tested by:

• Following the complete validation workflow step-by-step
• Testing multiple project types (console, class library, web API, xUnit tests)
• Verifying build times, test execution, and code formatting
• Ensuring all commands work correctly in the current environment

The instructions start with a firm directive to reference them first before falling back to search or bash commands, ensuring agents work efficiently with known-good workflows.

Fixes #23.

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