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

CLAUDE.md

@@ -363,6 +363,19 @@ docker compose down
 
 ---
 
+## Data Ownership Policy (Critical)
+
+- **All business data must be persisted server-side** (PostgreSQL via backend APIs).
+- The frontend must treat the backend as the single source of truth for entities, tasks,
+  generated preview messages/history, logs, and any other domain data.
+- The frontend must **not** persist business/domain data in browser storage (`localStorage`,
+  `sessionStorage`, `IndexedDB`) or call LLM providers directly.
+- The only browser-stored auth state is the backend-issued session token cookie (`httpOnly` JWT).
+- If a required endpoint does not exist yet, implement it in the backend first; do not add
+  frontend-side persistence workarounds.
+
+---
+
 ## Naming Conventions
 
 ### Backend