docs(policy): enforce server-side data ownership and backend LLM mediation

- clarify frontend may only rely on backend-issued session token cookie for auth

- forbid frontend browser storage for domain/business data

- require backend-mediated LLM calls across agent workflows

.github/agents/backend.agent.md

@@ -98,3 +98,5 @@ cd backend
 - DO NOT put business logic in controllers.
 - DO NOT put prompt construction logic outside `PromptBuilderService`.
 - DO NOT modify frontend code — your scope is `backend/` only.
+- DO enforce server-side persistence for all business/domain data; frontend must not be required to persist domain data.
+- DO model generated test-message history as backend-owned task-related data with referential integrity and cleanup on task deletion.