fix: initialize sortOrder when providing parent

Todos/Model/Task.swift

@@ -42,6 +42,7 @@ final class SubTask: Codable, Ordered {
   init(name: String, parent: Task? = nil) {
     self.name = name
     self.task = parent
+    self.sortOrder = parent?.subtasks.count ?? 0
   }
 
   func yaml(_ indent: Int = 0) -> String {
@@ -112,6 +113,7 @@ final class Task: Codable, Ordered {
     self.name = name
     self.project = parent
     self.category = parent?.category ?? ""
+    self.sortOrder = parent?.tasks.count ?? 0
   }
 
   func yaml(_ indent: Int = 0) -> String {