Add A-Frame architecture and Testing Without Mocks guidelines

Document James Shore's patterns for clean architecture:
- A-Frame: Logic and Infrastructure as independent peers
- Testing Without Mocks: state-based tests, Nullables, no mocking
- Testing approach by layer (Logic, Infrastructure, Application)

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Co-Authored-By: Claude <noreply@anthropic.com>

AGENTS.md

@@ -34,6 +34,56 @@ A pre-commit hook is provided that runs `mix format --check-formatted` and `mix
 - **Prefer small, focused commits** - each commit should represent a single logical change
 - **Write tests first** - follow test-driven development (TDD) where practical; write failing tests before implementing features
 - Use `mix precommit` alias when you are done with all changes and fix any pending issues
+
+### Architecture: A-Frame Pattern
+
+This project follows [James Shore's A-Frame Architecture](https://www.jamesshore.com/v2/projects/nullables/testing-without-mocks) where Logic and Infrastructure are independent peers coordinated at the Application layer:
+
+```
+   Application/UI     Values
+   /            \
+  V              V
+Logic   Infrastructure
+```
+
+- **Logic** - Pure business logic with no external dependencies; easily testable with state-based tests
+- **Infrastructure** - Wrappers around external systems (database, APIs, file system); one wrapper per external system
+- **Application** - Coordinates Logic and Infrastructure; thin layer that wires things together
+- **Values** - Immutable data structures passed between layers
+
+### Testing: Without Mocks
+
+Follow the "Testing Without Mocks" pattern language. Key principles:
+
+- **Avoid mocks** - They test implementation details and break when refactoring
+- **State-based tests** - Check output/state, not function calls
+- **Narrow tests** - Focus on specific functions/behaviors, not system-wide functionality
+- **Overlapping sociable tests** - Execute real dependencies; dependencies have their own thorough tests
+
+#### Nullables Pattern
+
+For infrastructure code, implement **Nullables** - production code with an off-switch for external communication:
+
+- Create a factory method that returns a "null" version for testing (disables external calls but preserves behavior)
+- Nullables have legitimate production uses (dry-run modes, cache warming)
+- Test Nullables like any production code
+
+#### Infrastructure Wrappers
+
+- One wrapper module per external system (Repo for database, HTTP clients for APIs)
+- Expose clean interfaces matching application needs, not third-party APIs
+- These are the **only** places where external communication occurs
+- Test with narrow integration tests against real systems (test database, etc.)
+
+#### Testing Approach by Layer
+
+- **Logic** - Pure state-based tests; no infrastructure needed
+- **Infrastructure wrappers** - Narrow integration tests with real external systems
+- **Application code** - Sociable tests with Nullable dependencies
+- **LiveViews/Controllers** - Use Phoenix test helpers with test database
+
+### Dependencies
+
 - Use the already included and available `:req` (`Req`) library for HTTP requests, **avoid** `:httpoison`, `:tesla`, and `:httpc`. Req is included by default and is the preferred HTTP client for Phoenix apps
 
 ### Phoenix v1.8 guidelines