feat(api): add parentTaskId support to task update and MCP create

Allow setting ParentTaskId when updating a task via the API (with
validation that the parent exists) and when creating a task via the
MCP create_task tool.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

TaskTracker.Core/DTOs/UpdateTaskRequest.cs

@@ -6,4 +6,5 @@ public class UpdateTaskRequest
     public string? Description { get; set; }
     public string? Category { get; set; }
     public int? EstimatedMinutes { get; set; }
+    public int? ParentTaskId { get; set; }
 }