feat(ui): warn when the running worker predates the selected repo's merged HEAD

Stamps ClaudeDo.Worker's build with its exact git SHA (SourceRevisionId ->
InformationalVersion) and exposes it via a new GetWorkerBuildInfo hub call.
For the currently selected list, the shell compares that SHA against the
list's git HEAD (GitService.IsAncestorAsync) and shows a persistent footer
banner -- never auto-clearing, never shown on an unrelated repo or when the
ancestry can't be determined -- so "verified against a merge" claims aren't
silently made against a stale process. No auto-restart; the banner just
offers the existing RestartWorkerCommand.

src/ClaudeDo.Ui/Views/MainWindow.axaml

@@ -258,5 +258,28 @@
       </StackPanel>
     </Border>
 
+    <!-- Stale-worker notice — bottom-right corner, above the update popup. Persistent
+         (no auto-dismiss timer): the running worker predates the selected list's merged
+         HEAD, so a restart is needed before "verified" claims against it are trustworthy. -->
+    <Border Grid.Row="0" Grid.RowSpan="3"
+            VerticalAlignment="Bottom" HorizontalAlignment="Right"
+            Margin="0,0,16,100"
+            ZIndex="100"
+            IsVisible="{Binding IsStaleWorkerBannerVisible}"
+            Background="{DynamicResource DeepBrush}"
+            BorderBrush="{DynamicResource LineBrush}"
+            BorderThickness="1"
+            CornerRadius="6"
+            Padding="14,10">
+      <StackPanel Spacing="8" Width="260">
+        <TextBlock Classes="body" Text="{loc:Tr shell.staleWorker.message}" TextWrapping="Wrap"/>
+        <StackPanel Orientation="Horizontal" Spacing="8" HorizontalAlignment="Right">
+          <Button Classes="btn"
+                  Content="{loc:Tr shell.menu.restartWorker}"
+                  Command="{Binding RestartWorkerCommand}"/>
+        </StackPanel>
+      </StackPanel>
+    </Border>
+
   </Grid>
 </Window>