Create Website Assistant Session

Open the Website workspace's assistant for this operator.

Returns the tenant's Website Agent + a conversation bound to (this user, this property), creating or resuming it. POST (not GET) because it mutates: resolving the session may create the conversation, and provision the agent.

Mirrors the guide's /assistant-session; _owned_property already folds in the tenant scope, the admin gate and the WEBSITE_CLONE flag, so this route cannot hand out an agent on a surface the person may not open.