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/MissionControlViewModel.cs

@@ -54,10 +54,13 @@ public sealed partial class MissionControlViewModel : ViewModelBase, IDisposable
     public ObservableCollection<QueuedTaskViewModel> Queued { get; } = new();
     public bool HasQueued => Queued.Count > 0;
 
-    public MissionControlViewModel(IDbContextFactory<ClaudeDoDbContext> dbFactory, IWorkerClient worker)
+    public UsagePillViewModel UsagePill { get; }
+
+    public MissionControlViewModel(IDbContextFactory<ClaudeDoDbContext> dbFactory, IWorkerClient worker, UsagePillViewModel usagePill)
     {
         _dbFactory = dbFactory;
         _worker = worker;
+        UsagePill = usagePill;
 
         ConPtySessions.CollectionChanged += OnConPtySessionsChanged;
         Panes.CollectionChanged += OnPanesChanged;