chore(claude-do): refactor(ui): DetailsIslandViewModel (1431 Zeilen) in Sektio

Kontext: src/ClaudeDo.Ui/ViewModels/Islands/DetailsIslandViewModel.cs ist mit 1431 Zeilen ein God-VM mit ~12 Concerns (Log-Streaming, Titel/Description-Editing, Subtasks, Child-Outcomes, Merge-Preview/-Targets, Diff, Agent-Settings-Overrides, Notes-Mode, Prep-Mode, Tabs, Session-Outcome/Roadblocks, Worktree-Info). Jedes neue Feature landet dort.

Änderungen — drei klar abgrenzbare Sektionen als ei

ClaudeDo-Task: 483e419f-1ec8-46ba-986b-8b90d6596b49

src/ClaudeDo.Ui/Views/Islands/DetailsIslandView.axaml

@@ -104,16 +104,16 @@
         <DockPanel>
           <Border DockPanel.Dock="Top" Padding="12,8">
             <Button Classes="btn primary"
-                    Command="{Binding PlanDayCommand}"
-                    IsEnabled="{Binding !IsPrepRunning}"
+                    Command="{Binding Prep.PlanDayCommand}"
+                    IsEnabled="{Binding !Prep.IsPrepRunning}"
                     Content="{loc:Tr details.planDay}"/>
           </Border>
           <Panel>
             <islands:SessionTerminalView
                 Margin="18,8,18,0"
-                Entries="{Binding PrepLog}" Label="daily-prep"
-                IsRunning="{Binding IsPrepRunning}"/>
-            <TextBlock IsVisible="{Binding ShowPrepEmptyState}"
+                Entries="{Binding Prep.PrepLog}" Label="daily-prep"
+                IsRunning="{Binding Prep.IsPrepRunning}"/>
+            <TextBlock IsVisible="{Binding Prep.ShowPrepEmptyState}"
                        HorizontalAlignment="Center" VerticalAlignment="Center"
                        Foreground="{DynamicResource TextMuteBrush}"
                        Text="{loc:Tr details.prepEmpty}"/>