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.

tests/question-tools.test.ts

@@ -317,8 +317,7 @@ describe("prepareQuestionArguments (the string-encoded options wobble)", () => {
 describe("teachExplainToolFor: the explain variant validates nodeId like the quiz", () => {
 	const validExplain = {
 		question: "Explain why the null result matters.",
-		explanation: "A stationary aether predicts a fringe shift.",
-		correctAnswer: "It ruled out the aether.",
+		correctAnswer: "It ruled out the aether — a stationary aether predicts a fringe shift, and none was seen.",
 		nodeId: "arith",
 	};
 
@@ -374,13 +373,11 @@ describe("the prober's explain: no nodeId field at all", () => {
 		assert.ok(!("nodeId" in probeExplainTool.parameters.properties));
 		const res = await boundaryParse(probeExplainTool, {
 			question: "Explain what a derivative measures.",
-			explanation: "It is instantaneous rate of change.",
-			correctAnswer: "The instantaneous rate of change.",
+			correctAnswer: "The instantaneous rate of change at a point — the limit the slopes converge to.",
 		});
 		assert.equal(res.success, true);
 		const empty = await boundaryParse(probeExplainTool, {
 			question: "q",
-			explanation: "e",
 			correctAnswer: " ",
 		});
 		assert.equal(empty.success, false);