feat(mcp): paths filter for get_task_diff, subset hint for preview_merge_set

get_task_diff gained an optional paths param (git pathspec, both stat and
full-diff modes) so a large multi-file task can be narrowed to the files
actually in question instead of shipping the whole diff.

preview_merge_set now also reports Subsets: pairs where one task's changed
files are a proper subset of another's in the same set, the strongest
available post-hoc signal that a task may be redundant with another.

src/ClaudeDo.Ui/ViewModels/Modals/DiffViewerViewModel.cs

@@ -162,9 +162,9 @@ public sealed partial class DiffViewerViewModel : ViewModelBase
         try
         {
             raw = FromCommitRange && BaseRef is not null && HeadCommit is not null
-                ? await _git.GetCommitRangeDiffAsync(WorktreePath, BaseRef, HeadCommit, ct)
+                ? await _git.GetCommitRangeDiffAsync(WorktreePath, BaseRef, HeadCommit, ct: ct)
                 : BaseRef is not null
-                    ? await _git.GetBranchDiffAsync(WorktreePath, BaseRef, ct)
+                    ? await _git.GetBranchDiffAsync(WorktreePath, BaseRef, ct: ct)
                     : await _git.GetDiffAsync(WorktreePath, ct);
         }
         catch (Exception ex)