docs: define repository pattern boundary

docs/ARCHITECTURE.md

@@ -97,6 +97,28 @@ Domain code should not depend on delivery mechanisms, databases, network
 clients, or framework-specific runtime concerns unless the project intentionally
 uses an architecture where that trade-off is accepted and recorded.
 
+## Persistence Boundary
+
+The default persistence boundary is the repository pattern.
+
+Use repositories to express domain-specific persistence operations without
+leaking database, ORM, file-system, or external storage details into application
+or domain logic.
+
+Default rules:
+
+- Application or domain-facing code depends on repository interfaces.
+- Infrastructure implements repository interfaces.
+- Repositories are named around domain concepts, not database tables.
+- Repositories expose intention-revealing operations, not generic CRUD by
+  default.
+- Transaction boundaries are explicit at the application service, unit of work,
+  or framework boundary selected by the project.
+
+If a project intentionally avoids repositories because the framework or
+architecture provides a better persistence boundary, record that decision in
+[DECISIONS.md](DECISIONS.md).
+
 ## Request Flow
 
 ```mermaid