This PR creates comprehensive GitHub Copilot instructions for the dotnet/arcade repository to help coding agents work effectively with this critical .NET infrastructure codebase.

The instructions provide detailed guidance on:

Core Infrastructure Setup

• Required .NET SDK: 10.0.100-preview.7.25372.107 (auto-installed via ./eng/common/dotnet.sh)
• Network Dependencies: Azure DevOps package feeds (dnceng organization) required for builds
• Build System: Uses Arcade SDK with MSBuild-based build orchestration

Build and Test Processes

• Build Commands: ./build.sh --restore --build (Linux/macOS) or Build.cmd (Windows)
• Test Commands: ./test.sh or Test.cmd with comprehensive unit and integration test suites
• Critical Timing: 45-90 minute builds, 15-30 minute test runs - includes explicit NEVER CANCEL warnings
• Timeout Requirements: 90+ minutes for builds, 30+ minutes for tests to prevent premature cancellation

Key Components Documented

• Arcade SDK: Core MSBuild SDK providing consistent build infrastructure
• Helix: Distributed testing infrastructure with 50+ repositories
• SignTool: Code signing automation and certificate management
• Build Tasks: Packaging, installers, workloads, and Visual Studio integration
• Testing Infrastructure: XUnit extensions, remote execution, and package validation

Validation Scenarios

• Complete build validation workflows with proper timeout handling
• Manual testing requirements for infrastructure changes
• Troubleshooting guidance for common network and SDK issues
• Artifact structure and output validation procedures

The instructions follow an imperative tone ("Run [command]", "Do not [action]") and emphasize the critical nature of timing and network dependencies since this repository provides infrastructure tooling used across 100+ .NET repositories.

Fixes #16040.

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