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/Controls/UsagePill.axaml

@@ -0,0 +1,25 @@
+<UserControl xmlns="https://github.com/avaloniaui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             xmlns:vm="using:ClaudeDo.Ui.ViewModels"
+             x:Class="ClaudeDo.Ui.Views.Controls.UsagePill"
+             x:DataType="vm:UsagePillViewModel">
+  <Button Command="{Binding OpenMonitorCommand}"
+          Background="Transparent" BorderThickness="0" Padding="0"
+          Cursor="Hand" VerticalAlignment="Center"
+          ToolTip.Tip="{Binding Tooltip}">
+    <StackPanel Orientation="Horizontal" Spacing="7" VerticalAlignment="Center">
+      <Ellipse Width="7" Height="7" Fill="{DynamicResource TextDimBrush}"
+               IsVisible="{Binding ShowNormalDot}"/>
+      <Ellipse Width="7" Height="7" Fill="{DynamicResource StatusReviewBrush}"
+               IsVisible="{Binding ShowWarnDot}"/>
+      <Ellipse Width="7" Height="7" Fill="{DynamicResource StatusErrorBrush}"
+               IsVisible="{Binding IsBlocked}"/>
+      <Ellipse Width="7" Height="7" Fill="{DynamicResource TextMuteBrush}"
+               IsVisible="{Binding ShowStaleDot}"/>
+      <TextBlock Classes="eyebrow"
+                 Text="{Binding Text, Converter={StaticResource UpperCase}}"
+                 LetterSpacing="1.4"
+                 VerticalAlignment="Center"/>
+    </StackPanel>
+  </Button>
+</UserControl>