Add ADR for clean architecture adoption

Author: James Blair

docs/adr/0002-adopt-clean-architecture.md

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+# 2. Adopt clean architecture
+
+Date: 2025-11-07
+
+## Status
+
+Accepted
+
+## Context
+
+Software products with non-trivial business logic and requirements for increased 
+flexibility benefit from an architecture that maximizes modularity and testability, 
+while minimizing the cost and risk of change. With this Summer EBT product, we seek to
+maximize reuse across states where possible, while maintaining flexibility for state-specific
+context and needs.
+
+## Decision
+
+We have agreed to implement the C#/.NET components of this product using the Clean 
+Architecture approached popularized by Steve Smith (ardalis). Our approach is heavily
+inspired by the content and resources Steve has published:
+- [Clean Architecture with ASP.NET Core 8 | .NET Conf 2023 (YouTube)](https://www.youtube.com/watch?v=yF9SwL0p0Y0)
+- [Clean Architecture With ASP.NET Core — Steve Smith (YouTube)](https://www.youtube.com/watch?v=qOwB8PxOqC0)
+- [Clean Architecture with ASP.NET Core — Steve Smith (Blog Post)](https://ardalis.com/clean-architecture-asp-net-core/)
+- [Clean Architecture Sample Repository](https://github.com/ardalis/CleanArchitecture)
+
+## Consequences
+
+There is a small learning curve to building with this pattern for contributor who
+are unfamiliar with the approach, but overall the code for the product becomes significantly
+more maintainable, with core business logic easy to unit test, and a reduced risk from change.