Set or clear a member's monthly limit on workspace wallet spending [write, owner or admin]

The limit counts workspace-wallet charges attributed to that member in the calendar month (UTC): the admitted sender of each turn or consumption, including helpers. Historical charges without a recorded sender and work outside an active turn count against the session starter. At the limit a message to a session the workspace pays for, a helper, a handoff and a fork are refused with 402 member_limit_reached, and a new session is created waiting; other members are not affected. monthly_limit null removes the limit. When the new limit lets the member spend again their waiting tasks continue by themselves: resumed counts the messages that were sent on.