docs: define repository pattern boundary

examples/ARCHITECTURE.example.md

@@ -78,6 +78,19 @@ atlas-desk/
 - Domain rules do not depend on HTTP, Vue, or database APIs.
 - Infrastructure implementations depend inward on application contracts.
 
+## Persistence Boundary
+
+Atlas Desk uses the repository pattern for persistence boundaries.
+
+- Repository interfaces live in the owning backend module.
+- Repository implementations live in the module's infrastructure or persistence
+  folder.
+- Repositories are domain-specific, such as `RequestRepository` and
+  `UserRepository`.
+- Application services coordinate transactions for workflows that write multiple
+  records, such as assignment plus audit event creation.
+- Generic CRUD repositories are not used because they hide workflow intent.
+
 ## Request Flow
 
 ```mermaid