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.Worker/Lifecycle/TaskMergeService.cs

@@ -325,7 +325,9 @@ public sealed class TaskMergeService
             : targetBranch;
 
         // MergeAsync transitions the task WaitingForReview -> Done on a successful merge.
-        return await MergeAsync(taskId, target, removeWorktree: false, $"Merge {wt.BranchName}", ct);
+        // Remove the worktree on approve (matching the unit-merge path) so merged
+        // worktrees don't pile up; the merge commit on the target branch is the record.
+        return await MergeAsync(taskId, target, removeWorktree: true, $"Merge {wt.BranchName}", ct);
     }
 
     private static MergeResult Blocked(string reason) =>