[docs] document restrict_to_user_objects for MCP agent isolation

doc/user/content/integrations/mcp-server/mcp-agent.md

@@ -139,6 +139,33 @@ as needed.
 If any privilege is missing, the data product will not appear in the agent's
 tool list.
 
+### Restricting access to user objects only {#restrict-to-user-objects}
+
+By default, a role can query any object it has been granted `SELECT` on,
+including system catalog views. To prevent an agent role from reading system
+catalog tables (`mz_catalog`, `mz_internal`, `pg_catalog`,
+`information_schema`), a superuser can set the `restrict_to_user_objects` role
+default:
+
+```mzsql
+ALTER ROLE mcp_agent SET restrict_to_user_objects = true;
+```
+
+This setting takes effect on the next connection. Once active:
+
+- Queries referencing system catalog objects are rejected with a permission
+  error.
+- Data product discovery (`get_data_products`, `get_data_product_details`,
+  `read_data_product`) continues to work normally.
+- The restriction cannot be bypassed by the role itself; only a superuser can
+  change or remove it.
+
+To remove the restriction (as superuser):
+
+```mzsql
+ALTER ROLE mcp_agent RESET restrict_to_user_objects;
+```
+
 ## Define and document data products for discovery
 
 The MCP server allows agents to discover and query documented (i.e.,