List runtime instances serving an agent

Read-only view of the worker processes currently registered for a logical agent. Rows exist only for multi-runtime registrations (MULTI_RUNTIME_ENABLED), so the list is empty in the default configuration. isLive combines status with lastSeenAt freshness against the server's staleness cutoff (staleThresholdMinutes); reportedSlots is each process's self-reported capacity, distinct from the agent's logical maxTasks policy.