feat(ui): single approve action merges the whole unit

Approve & Merge is now the only review+merge entry. For a parent with
children it drives the unit merge via the worker (conflicts still surface
through the existing PlanningMergeConflict dialog); the separate Merge All
Subtasks button, MergeAllCommand, CanMergeAll plumbing, and the dead
MergeAllPlanningAsync client method are removed. Combined-diff preview and
conflict continue/abort are kept.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/ClaudeDo.Ui/Views/Islands/Detail/WorkConsole.axaml

@@ -305,17 +305,7 @@
               </Button>
               <Button Classes="btn" Content="Review Combined Diff" Margin="0,0,8,8"
                       Command="{Binding ReviewCombinedDiffCommand}" />
-              <Button Classes="btn accent" Content="Merge All Subtasks" Margin="0,0,0,8"
-                      Command="{Binding MergeAllCommand}"
-                      IsEnabled="{Binding CanMergeAll}"
-                      ToolTip.Tip="{Binding MergeAllDisabledReason}" />
             </WrapPanel>
-
-            <TextBlock Text="{Binding MergeAllError}"
-                       Foreground="{DynamicResource BloodBrush}"
-                       TextWrapping="Wrap"
-                       IsVisible="{Binding MergeAllError,
-                           Converter={x:Static ObjectConverters.IsNotNull}}" />
           </StackPanel>
         </ScrollViewer>