Add `grading_notes` to problem YAML

Now that `rubric` has been misappropriated as the template for the grading box, we need someplace else for purely internal notes on grading.

cc @yslan

Related but distinct: #1081

Locate the problem YAML schema or parser and trace how the existing rubric field is represented and used for the grading box. Read the related issue #1081 for context, then verify that a separate grading_notes field supports purely internal grading notes without changing rubric behavior.

Written by the indexing model from the issue text.