feat(ui): richer diff viewer + surface child roadblocks on parents

- UnifiedDiffParser detects added/deleted/renamed/binary files; diff
  modal shows a file list, binary/empty placeholders, and can diff a
  merged task by commit range after its worktree is gone
- DetailsIslandViewModel flags children needing attention (failed,
  cancelled, awaiting review, or with roadblocks) on the parent
- GitService gains worktree head-commit/range support; planning chain,
  merge orchestration, and session manager tweaks with updated tests
- refresh app/installer/worker icons

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

src/ClaudeDo.Ui/Design/IslandStyles.axaml

@@ -574,6 +574,13 @@
   <Style Selector="Border[Tag=?] > TextBlock">
     <Setter Property="Foreground" Value="{StaticResource TextFaintBrush}"/>
   </Style>
+  <!-- R → rename (sage) -->
+  <Style Selector="Border[Tag=R]">
+    <Setter Property="Background" Value="#268B9D7A"/>
+  </Style>
+  <Style Selector="Border[Tag=R] > TextBlock">
+    <Setter Property="Foreground" Value="{StaticResource SageBrush}"/>
+  </Style>
 
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