docs: map existing codebase
- STACK.md - Technologies and dependencies (.NET 4.7.2, NuGet, Costura.Fody)
- ARCHITECTURE.md - Layered CLI with ICncMachine factory pattern
- STRUCTURE.md - Directory layout and where to add new code
- CONVENTIONS.md - C# style, naming, result wrapper pattern
- TESTING.md - NUnit 3 + Moq, stubs, ignored FOCAS tests
- INTEGRATIONS.md - FOCAS, LSV2, FTP, RSA licensing
- CONCERNS.md - Blocking async, silent catch, no logging, Windows-only
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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