feat(ui): add usage pill to footer and mission control header

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.

src/ClaudeDo.Ui/ViewModels/IslandsShellViewModel.cs

@@ -21,6 +21,7 @@ public sealed partial class IslandsShellViewModel : ViewModelBase, IDisposable
     public DetailsIslandViewModel? Details { get; }
     public IWorkerClient? Worker { get; }
     public MissionControlViewModel? MissionControl { get; }
+    public UsagePillViewModel? UsagePill { get; }
     public UpdateCheckService UpdateCheck => _updateCheck;
 
     public string ConnectionText =>
@@ -208,10 +209,12 @@ public sealed partial class IslandsShellViewModel : ViewModelBase, IDisposable
         Func<WeeklyReportModalViewModel> weeklyReportVmFactory,
         Func<MergeModalViewModel> mergeVmFactory,
         Func<RepoImportModalViewModel> repoImportVmFactory,
-        MissionControlViewModel missionControl)
+        MissionControlViewModel missionControl,
+        UsagePillViewModel usagePill)
     {
         Lists = lists; Tasks = tasks; Details = details; Worker = worker;
         MissionControl = missionControl;
+        UsagePill = usagePill;
         MissionControl.OpenInApp = id => _ = RevealTaskAsync(id);
         MissionControl.OpenSettingsRequested = () => Lists.OpenSettingsCommand.Execute(null);
         MissionControl.ErrorReported += FlashFooterError;