Adds the IWorkerClient/WorkerClient usage surface (GetUsageSnapshot, GetModelUsage, GetTaskUsage, UsageUpdated event) and a shared UsagePillViewModel hosted once in IslandsShellViewModel (footer) and once in MissionControlViewModel (header), showing "5h X% · 7d Y%" with warn/blocked/stale states via existing design tokens. The OpenMonitorCommand is wired but currently a no-op, pending the usage monitor modal.