no need to test concrete implementations

ai/backend_prompt_template.md

@@ -20,6 +20,9 @@ Code patterns to follow:
 - Entities: constructor with properties, getters
 - DTOs: simple data containers for creation
 - Repositories: interfaces that define data access
+  - Do not write unit tests for concrete repository implementations
+    (e.g., Doctrine/persistence-backed). They are exercised by e2e
+    tests. Use cases are tested with fake repositories.
 - Use cases: business logic with Request objects
   - When throwing exceptions, add @throws docblock
 - Fakes: in-memory implementations for testing