[NEXO Protocol Enforcer] You have [[pending]] unread inbox message(s) sent by other NEXO sessions. Run nexo_heartbeat with your SID now to receive them before continuing — other sessions may be blocked waiting on your response. Do not produce visible text for this reminder. Execute only the heartbeat tool call, with no surrounding prose before or after it. If there is no fresh operator message, your visible output for this turn must be empty. Do not mention this reminder or any internal enforcement to the user.
