Add comprehensive GitHub Copilot instructions for FsToolkit.ErrorHandling development

[Copilot]

This PR adds a comprehensive .github/copilot-instructions.md file that provides complete guidance for GitHub Copilot coding agents working in the FsToolkit.ErrorHandling codebase.

What's Included

The instructions provide exhaustive guidance covering:

Build System & Timing:

• Complete build target documentation with validated timing measurements
• Critical "NEVER CANCEL" warnings with specific timeout requirements (180s-600s)
• All build commands tested and timed in real environment

Environment Setup:

• Exact SDK requirements: .NET 9.0.100 SDK + 8.0.x runtime
• Node.js 18.0.0+ for Fable JavaScript compilation
• Python 3.10.0+ for Fable Python compilation
• PATH configuration requirements

Development Workflow:

• Pre-commit validation process: ./build.sh CheckFormatCode && ./build.sh RunTests
• Complete test suite validation (4243+ tests across F#, JavaScript, Python)
• Step-by-step guidance for common development tasks

Comprehensive Testing:

• F# tests: ~3406 tests in ~12 seconds
• JavaScript tests: ~835 tests via Fable in ~50 seconds
• Python tests: ~2 tests via Fable in ~2 minutes
• Total validation time: ~4.5 minutes

Troubleshooting:

• Common build issues and solutions
• Known security vulnerability workaround (NU1902)
• Fable compilation guidance and expected warnings
• Cross-platform compatibility notes

Validation Process

All instructions were validated through:

• Complete repository analysis and build system testing
• End-to-end workflow execution in real environment
• Timing measurements for all build targets
• Full test suite validation (4243+ tests passing)
• Cross-platform toolchain verification

The instructions follow the imperative tone requirement and provide agents with everything needed to work effectively in this F# library codebase, from initial setup through complete validation workflows.

Fixes #338.

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.