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/Views/MissionControl/MissionControlView.axaml

@@ -3,6 +3,7 @@
              xmlns:vm="using:ClaudeDo.Ui.ViewModels"
              xmlns:vmm="using:ClaudeDo.Ui.ViewModels.MissionControl"
              xmlns:mc="using:ClaudeDo.Ui.Views.MissionControl"
+             xmlns:controls="using:ClaudeDo.Ui.Views.Controls"
              xmlns:loc="using:ClaudeDo.Ui.Localization"
              x:DataType="vm:MissionControlViewModel"
              x:Class="ClaudeDo.Ui.Views.MissionControl.MissionControlView">
@@ -25,6 +26,9 @@
                    LetterSpacing="1.4" VerticalAlignment="Center" />
         <StackPanel Grid.Column="1" Orientation="Horizontal" Spacing="8"
                     VerticalAlignment="Center">
+          <controls:UsagePill DataContext="{Binding UsagePill}"
+                               Margin="0,0,4,0"
+                               VerticalAlignment="Center"/>
           <Button Classes="icon-btn"
                   Click="OnNewSessionClicked"
                   ToolTip.Tip="{loc:Tr missionControl.newSession}">