Add streamlined GitHub Copilot instructions for MSBuild repository development

[Copilot]

This PR adds comprehensive GitHub Copilot instructions to help coding agents work effectively with the MSBuild repository. The instructions provide essential guidance for building, testing, and developing with MSBuild across Windows, macOS, and Linux platforms.

Key Features

Cross-Platform Build Support:

• Complete build instructions for both Windows (build.cmd) and Unix systems (build.sh)
• Platform-specific bootstrap environment setup scripts
• Precise timing guidance (2-3 minutes for full build, 9 minutes for tests)
• Timeout specifications to prevent premature cancellation of long-running operations

Performance and Coding Standards:

• Emphasis on MSBuild's performance-first approach (minimize allocations, avoid LINQ)
• Modern C# features guidance (C# 13, collection expressions, prefer [] over new Type[])
• Testing requirements with xUnit and Shouldly assertions
• Code style conventions matching .NET Runtime guidelines
• Nullable type guidance for new code without aggressive refactoring

Streamlined Development Workflow:

• Correct MSBuild executable paths using the bootstrap environment (artifacts/bin/bootstrap/core/MSBuild.dll)
• Simplified build commands without redundant flags
• Focus on individual test execution over full test suite
• Essential file structure and key project understanding

Practical Troubleshooting:

• Solutions for common SDK resolution issues
• Guidance on using repository sample projects for testing
• Certificate error handling for external projects

The instructions have been refined based on maintainer feedback to focus on the most essential information for effective development while removing sections that add unnecessary complexity to the context window.

Fixes #12338.

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✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[dotnet-policy-service]

Hello @@copilot, I noticed that you’re changing an .swr file or any file under src/Package/MSBuild.VSSetup.. Please make sure to validate this change by an experimental VS insertion. This is accomplished by pushing to an exp/* branch, which requires write permissions to this repo.

[rainersigwald]

This is what I've been using locally, but I haven't gotten enough experimentation time with it to know how much it helps:

This repo contains the code for the MSBuild build engine, including its public C# API, its internal implementation of the MSBuild programming language, and core targets and tasks used for builds for .NET and Visual Studio.

Performance is very important--minimize allocations, avoid LINQ, and use the most efficient algorithms possible. The code should be easy to read and understand, but performance is the top priority.

The code is written in C# and should follow the .NET coding conventions. Use the latest C# features where appropriate, including C# 13 features and especially collection expressions--prefer [] to new Type[].

You should generally match the style of surrounding code when making edits, but if making a substantial change, you can modernize more aggressively.

Generate tests for new codepaths, and add tests for any bugs you fix. Use the existing test framework, which is xUnit with Shouldly assertions. Use Shouldly assertions for all assertions in modified code, even if the file is predominantly using xUnit assertions.

[YuliiaKovalova]

This is what I've been using locally, but I haven't gotten enough experimentation time with it to know how much it helps:

This repo contains the code for the MSBuild build engine, including its public C# API, its internal implementation of the MSBuild programming language, and core targets and tasks used for builds for .NET and Visual Studio.
Performance is very important--minimize allocations, avoid LINQ, and use the most efficient algorithms possible. The code should be easy to read and understand, but performance is the top priority.
The code is written in C# and should follow the .NET coding conventions. Use the latest C# features where appropriate, including C# 13 features and especially collection expressions--prefer [] to new Type[].
You should generally match the style of surrounding code when making edits, but if making a substantial change, you can modernize more aggressively.
Generate tests for new codepaths, and add tests for any bugs you fix. Use the existing test framework, which is xUnit with Shouldly assertions. Use Shouldly assertions for all assertions in modified code, even if the file is predominantly using xUnit assertions.

@copilot check it out and apply

[Copilot]

This is what I've been using locally, but I haven't gotten enough experimentation time with it to know how much it helps:

This repo contains the code for the MSBuild build engine, including...

Applied @rainersigwald's guidance from the previous comment. Added:

• Performance emphasis (minimize allocations, avoid LINQ, efficient algorithms)
• Modern C# features preference (C# 13, collection expressions, [] over new Type[])
• Testing requirements (xUnit with Shouldly assertions)
• Code style guidance (match surrounding code, modernize on substantial changes)
• Repository description focusing on MSBuild build engine and API
  All integrated into the comprehensive instructions. 095a415

[Copilot]

Pull Request Overview

This PR introduces comprehensive GitHub Copilot instructions to enhance AI-assisted development in the MSBuild repository. It provides detailed guidance for building, testing, and developing MSBuild across Windows, macOS, and Linux platforms, with emphasis on performance-first coding practices and proper development workflows.

Key changes:

• Cross-platform build and bootstrap instructions with specific timing guidance
• Performance-focused coding standards emphasizing allocation minimization and modern C# features
• Comprehensive troubleshooting section addressing common development environment issues

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

[JanProvaznik]

Few edits that I think would be beneficial. I'm curious to see if this file will help the agent. I wouldn't keep this for much longer in review, we can always improve or turn it off.

[Copilot]

This is what I've been using locally, but I haven't gotten enough experimentation time with it to know how much it helps:

This repo contains the code for the MSBuild build engine, including...

Applied @rainersigwald's guidance. The instructions now include performance emphasis (minimize allocations, avoid LINQ), modern C# features preference (C# 13, collection expressions), testing requirements (xUnit with Shouldly assertions), and code style guidance. f96a492