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>

src/ClaudeDo.Ui/Views/Controls/UsagePill.axaml.cs

@@ -0,0 +1,8 @@
+using Avalonia.Controls;
+
+namespace ClaudeDo.Ui.Views.Controls;
+
+public partial class UsagePill : UserControl
+{
+    public UsagePill() => InitializeComponent();
+}

src/ClaudeDo.Ui/Views/MainWindow.axaml

@@ -2,6 +2,7 @@
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:vm="using:ClaudeDo.Ui.ViewModels"
         xmlns:islands="using:ClaudeDo.Ui.Views.Islands"
+        xmlns:controls="using:ClaudeDo.Ui.Views.Controls"
         xmlns:converters="using:ClaudeDo.Ui.Converters"
         xmlns:loc="using:ClaudeDo.Ui.Localization"
         x:Class="ClaudeDo.Ui.Views.MainWindow"
@@ -184,6 +185,12 @@
           </StackPanel>
         </Button>
 
+        <!-- Left: usage pill (click opens the usage monitor, wired in a later subtask) -->
+        <controls:UsagePill DockPanel.Dock="Left"
+                             DataContext="{Binding UsagePill}"
+                             Margin="14,0,0,0"
+                             VerticalAlignment="Center"/>
+
         <!-- Right: worker log line — click to open the Log Visualizer overlay -->
         <Button DockPanel.Dock="Right"
                 Command="{Binding OpenLogVisualizerCommand}"

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}">