Set or clear a deck coaching note (Markdown)

Write. Same write as PATCH /dev/decks/{deckId} with deckNote. Body is { deckId, deckNote }. deckNote null clears it. Coaching notes are instructions for how an agent should quiz and coach. They are not the answer, not graded content, and not the quiz prompt. A deck note applies to every card in that deck. A card note is optional and additive with the deck note. Retrieve responses (listDue, upcoming-new, inventory, getCard, reveal, and agent session start/next/grade) include deckNote and cardNote so one call carries coaching context. Scripture / dictation: The learner needs every word remembered correctly, but often answers via dictation where punctuation gets messy. They set a deck note instructing the agent to check every word exactly, but not fail on punctuation differences, and to include that reason when coaching. This is only an example — notes can be any coaching preference for how agents should quiz/coach for that deck (or a specific card).