explain refinements: no opt-out row, one committed field (owner 2026-09-27)

Two follow-up rulings on the explain tool:

1. No 'I don't know' row — not needed in free recall. If he doesn't
   know, he says so in his own words; his text is the only answer
   channel. Dock: text mode is rowless again (the composer IS the
   surface); doctrine and the shared prompt chunk updated; the format
   branch and card revert to quiz-only dontKnow.

2. explanation + correctAnswer collapse into ONE field — the separate
   explanation was quiz remnant (MC needs the why outside its options;
   a free-text commitment carries its own reasoning). Schema is now
   {question, correctAnswer, details?}; the reveal shows the single
   committed answer; no explanation snapshots anywhere on the explain
   path.

1047/1047 tests, typecheck/lint/build green.

src/agent/prompts/shared/20-question-tools.md

@@ -4,9 +4,8 @@ Two instruments. The quiz tool (its exact name depends on the quizzing mechanic
 
 ### Quiz checks — shared doctrine
 
-- `explanation` (required) says why the correct answer is correct. It comes BEFORE `correctAnswer` in the call by design: reason it out first, then commit to the answer the reasoning supports.
-- An "I don't know" choice is always available to the learner — never author your own opt-out. A dontKnow result is a genuine gap, not a wrong answer.
-- Answers may carry a free-text `note` — read it; it says what he was thinking.
+- Multiple choice carries a separate `explanation` (why the correct option is correct, written BEFORE `correctAnswer`: reason it out first, then commit). Free recall has ONE field — `correctAnswer`, the committed answer with its reasoning inside.
+- Option-style answers may carry a free-text `note` — read it; it says what he was thinking.
 - ONE question per turn: ask, then end your turn — his answer determines the next.
 
 ### `ask_user_question`