feat(ui): session-skills empty-state + neutral subtask terminology

Show an explanatory empty-state under the install row when no skills are
installed. Rename the unified-parent status/labels from "Improvements" to
neutral "Subtasks" (waitingForChildren, agentStatus.children,
childOutcomesLabel) since WaitingForChildren now covers planning too.

src/ClaudeDo.Ui/Views/Modals/SettingsModalView.axaml

@@ -413,6 +413,9 @@
                     </DataTemplate>
                   </ItemsControl.ItemTemplate>
                 </ItemsControl>
+                <TextBlock Classes="meta" Opacity="0.6" TextWrapping="Wrap"
+                           Text="{loc:Tr settings.skills.emptyState}"
+                           IsVisible="{Binding SessionSkills.IsEmpty}"/>
               </StackPanel>
 
               <TextBlock Classes="meta" Text="{Binding SessionSkills.StatusMessage}"