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.App/Program.cs

@@ -159,10 +159,13 @@ sealed class Program
                 sp,
                 sp.GetRequiredService<INotesApi>(),
                 sp.GetRequiredService<IMergeCoordinator>()));
+        sc.AddSingleton<UsagePillViewModel>(sp =>
+            new UsagePillViewModel(sp.GetRequiredService<IWorkerClient>()));
         sc.AddSingleton<MissionControlViewModel>(sp =>
             new MissionControlViewModel(
                 sp.GetRequiredService<IDbContextFactory<ClaudeDoDbContext>>(),
-                sp.GetRequiredService<IWorkerClient>()));
+                sp.GetRequiredService<IWorkerClient>(),
+                sp.GetRequiredService<UsagePillViewModel>()));
         sc.AddSingleton<IslandsShellViewModel>(sp =>
         {
             var shell = ActivatorUtilities.CreateInstance<IslandsShellViewModel>(sp);