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/ViewModels/Modals/Settings/SessionSkillsSettingsTabViewModel.cs

@@ -13,6 +13,7 @@ public sealed partial class SessionSkillsSettingsTabViewModel : ViewModelBase
     [ObservableProperty] private string _installUrl = "";
     [ObservableProperty] private string _statusMessage = "";
     [ObservableProperty] private bool _isBusy;
+    [ObservableProperty] private bool _isEmpty = true;
 
     public ObservableCollection<SessionSkillDto> Skills { get; } = new();
 
@@ -26,6 +27,7 @@ public sealed partial class SessionSkillsSettingsTabViewModel : ViewModelBase
             var skills = await _worker.GetSessionSkillsAsync();
             Skills.Clear();
             foreach (var s in skills) Skills.Add(s);
+            IsEmpty = Skills.Count == 0;
         }
         finally { IsBusy = false; }
     }