refactor(tasks): route UI quick-add through TaskRepository.AddAsync

Drops the append-SortOrder query duplicated inline in TasksIslandViewModel.AddAsync; the repository (already used by MCP AddTask) is now the single home for the create+SortOrder invariant. Sets Status=Idle explicitly for parity.

src/ClaudeDo.Ui/ViewModels/Islands/TasksIslandViewModel.cs

@@ -408,20 +408,15 @@ public sealed partial class TasksIslandViewModel : ViewModelBase, IDisposable
         if (string.IsNullOrWhiteSpace(NewTaskTitle) || _currentList?.Kind != ListKind.User) return;
         var listId = _currentList.Id["user:".Length..];
         await using var db = await _dbFactory.CreateDbContextAsync();
-        var maxSort = await db.Tasks
-            .Where(t => t.ListId == listId)
-            .Select(t => (int?)t.SortOrder)
-            .MaxAsync();
         var entity = new TaskEntity
         {
             Id        = Guid.NewGuid().ToString("N"),
             ListId    = listId,
             Title     = NewTaskTitle.Trim(),
+            Status    = TaskStatus.Idle,
             CreatedAt = DateTime.UtcNow,
-            SortOrder = (maxSort ?? -1) + 1,
         };
-        db.Tasks.Add(entity);
-        await db.SaveChangesAsync();
+        await new TaskRepository(db).AddAsync(entity);
         var row = TaskRowViewModel.FromEntity(entity);
         row.ShowListChip = _currentList?.Kind == ListKind.Virtual;
         Items.Add(row);