fix(prompt): drop HOUSE_STYLE — the shipped prompt now says it itself

HOUSE_STYLE was appended after tutor-system.md because the old prompt
never stated which language to teach in, and gpt-oss-20b delivered a
unit-1.1 lesson in Korean. The 16 Sep prompt states the rule outright,
twice — "everything you SAY is in English" — and the client now has
proseIsKorean() to send a Korean explanation back. The appendix said
itself that if the shipped prompt ever covered this, it should go.

{{VARIETY}} also learns the fifth exercise type, recall.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app/src/domain/gate.ts

@@ -112,8 +112,8 @@ export function gateFor({ progress, bandQuery }: GateInputs): Gate {
 
 /* ── the prompt ──────────────────────────────────────────────────── */
 
-/** The four exercise types, so {{VARIETY}} can ask for a different one. */
+/** The five exercise types, so {{VARIETY}} can ask for a different one. */
-const TASK_TYPES = ["translate", "match", "build", "choice"] as const;
+const TASK_TYPES = ["translate", "recall", "match", "build", "choice"] as const;
 export type TaskKind = (typeof TASK_TYPES)[number];
 
 export const FOCUS_MODES = {
@@ -150,34 +150,6 @@ export interface PromptInputs {
   focus: FocusMode;
 }
 
-/**
- * Assemble the system prompt. The template is prompt/tutor-system.md,
- * shipped unchanged — this fills its three placeholders and nothing else.
- */
-/* Appended by the app, after the shipped prompt — not an edit to it.
-
-   tutor-system.md ships unchanged. Two things it leaves to inference, which
-   a strong model supplies on its own and a small local one does not:
-
-   THE LANGUAGE OF EXPLANATION is never actually stated. The prompt says
-   "한글 and English only", but that rule is about retiring romanization, not
-   about which language to teach in. Everything else only implies it —
-   English translations, English glosses. gpt-oss-20b read the room
-   differently and delivered a whole grammar lesson in Korean, to a student
-   on unit 1.1 who cannot yet read it.
-
-   NO MARKDOWN is stated outright at line 96, and was ignored anyway. It is
-   restated here because the consequence is invisible to the model: the app
-   renders **bold** and nothing else, so a table arrives as rows of literal
-   pipes and a heading as literal hashes.
-
-   Both are reinforcement, never contradiction. If the shipped prompt and
-   this ever disagree, the shipped prompt is right and this should go. */
-const HOUSE_STYLE = `
-════ HOUSE STYLE ════
-Explain in English. Korean is the material — words, examples, exercise lines, anything you are asking him to read. It is not the language you teach in. He is a beginner working through the writing system; an explanation he cannot read teaches him nothing.
-Plain text only. No tables, headings, bullet characters or code fences — the app renders **bold** and nothing else, so anything else reaches him as literal pipes and hashes.`;
-
 /**
  * Substitute one placeholder — the one standing alone on its own line.
  *
@@ -207,11 +179,15 @@ function fill(template: string, name: string, value: string): string {
   return template.replace(token, () => value);
 }
 
+/**
+ * Assemble the system prompt. The template is prompt/tutor-system.md,
+ * shipped unchanged — this fills its three placeholders and nothing else.
+ */
 export function assemblePrompt({ template, gate, recent, focus }: PromptInputs): string {
   let out = fill(template, "GATE", renderGate(gate));
   out = fill(out, "VARIETY", varietyLine(recent));
   out = fill(out, "FOCUS", focusLine(focus));
-  return out + HOUSE_STYLE;
+  return out;
 }
 
 export { renderGate, REFERENCE_BAND };