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

[Copilot]

The repository had no onboarding documentation, making it difficult for a Copilot coding agent (or any new contributor) to quickly understand how to build, test, and extend the codebase.

What's included

• Project orientation — what Lasy is, its interface-based abstraction model, and the three DB backends (RealDB, FakeDB, FileDB)
• Annotated directory tree — every significant file with a one-line description
• Build & test commands — MSBuild invocations, NUnit usage, and which tests require a live SQL Server
• Core design patterns — dictionary data model, Analyzer pattern, extension method conventions, Nvelope-backed metadata caching
• Known issues & workarounds:
  • Nvelope must be cloned as a sibling repo at ../../Nvelope/ (relative path project reference)
  • RealDBTests require a SQL Server instance; skip in CI-less environments
  • NUnit 2.5.10 is checked in under lib/; upgrading to v3 is a breaking change
  • FileDB parses fixed-width .rpt exports — column alignment is significant
  • No CI/CD pipeline exists
• New feature checklist — ordered steps for adding a DB operation across interfaces, implementations, and tests

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