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/ViewModels/Modals/MergeModalViewModel.cs

@@ -28,6 +28,10 @@ public sealed partial class MergeModalViewModel : ViewModelBase
 
     public Action? CloseAction { get; set; }
 
+    /// True once a merge has succeeded — lets the caller (e.g. the diff window)
+    /// close itself after this modal closes.
+    public bool Merged { get; private set; }
+
     public MergeModalViewModel(WorkerClient worker)
     {
         _worker = worker;
@@ -80,6 +84,7 @@ public sealed partial class MergeModalViewModel : ViewModelBase
             switch (result.Status)
             {
                 case "merged":
+                    Merged = true;
                     SuccessMessage = result.ErrorMessage is not null
                         ? $"Merged with warning: {result.ErrorMessage}"
                         : Loc.T("vm.merge.merged");