Add .github/copilot-instructions.md for agent onboarding

[Copilot]

Adds a copilot-instructions.md to give a coding agent the context needed to work efficiently in this repo without repeated exploration.

Covered

• Project overview — .NET Framework 4.6.1/4.0 only; no .NET Core; NuGet package v1.1.0.2
• Build system — Windows-only Rake + MSBuild pipeline; rake build/test/coverage; MSBuild direct invocation; no dotnet CLI support
• Testing — NUnit 2.6.3; how to run via Rake or nunit-console.exe; test file naming and structure conventions
• Code style — Extension method pattern, XML doc requirements, FxCop/StyleCop suppression pattern, namespace conventions
• Module map — Tables mapping all major source files/classes to descriptions across every namespace
• Known quirks — v1.1 breaking changes (DateTime locale, Enum strictness), dual-project build (src/Nvelope-net4 links same sources), StringExtensions.Print() dispatch quirk, checked-in packages/, strong-name signing requirement
• Dependency rules — No external runtime NuGet deps; lists required build tools
• Dev workflow — Step-by-step: find source → find test → edit → build → test → verify both framework targets

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⌨️ Start Copilot coding agent tasks without leaving your editor — available in VS Code, Visual Studio, JetBrains IDEs and Eclipse.