v0 scaffold: teacher runtime (hermetic pi SDK), data model, ledgers, normalizer

- teacher/system-prompt.md: teach methodology + quiz/ask guidelines folded in
- extensions/: quiz.ts + ask-user-question.ts (hermetic copies)
- src/teacher/create-session.ts: isolated agent (noTools builtin, own agentDir/sessions/prompt)
- src/schema.ts + ledger.ts: append-only ledgers, known-vs-understood data model
- src/extract/normalize.ts: session JSONL -> canonical doc (verified on real session)
- extractor/merger/derive/what-next: specified stubs

src/ledger.ts

@@ -0,0 +1,68 @@
+/**
+ * Append-only JSONL ledgers. Same philosophy as the design: the ledgers are
+ * the source of truth; everything else is derived and recomputable.
+ */
+
+import * as fs from "node:fs";
+import * as path from "node:path";
+import { randomBytes } from "node:crypto";
+import { LEDGER_FILES } from "../config.ts";
+
+export function newId(prefix: string): string {
+	return `${prefix}_${randomBytes(5).toString("hex")}`;
+}
+
+export function append<T>(file: string, record: T): void {
+	fs.mkdirSync(path.dirname(file), { recursive: true });
+	fs.appendFileSync(file, JSON.stringify(record) + "\n", "utf-8");
+}
+
+export function readAll<T>(file: string): T[] {
+	if (!fs.existsSync(file)) return [];
+	const out: T[] = [];
+	for (const line of fs.readFileSync(file, "utf-8").split("\n")) {
+		const trimmed = line.trim();
+		if (trimmed.length > 0) out.push(JSON.parse(trimmed) as T);
+	}
+	return out;
+}
+
+// Typed helpers ───────────────────────────────────────────────────────────────
+
+import type {
+	GraphEdge,
+	GraphNode,
+	KnowledgeEvent,
+	SessionMapping,
+	SessionRecord,
+} from "./schema.ts";
+
+export const ledgers = {
+	appendSession(record: SessionRecord) {
+		// Session records are unique per id — skip if already ingested.
+		if (readAll<SessionRecord>(LEDGER_FILES.sessions).some((s) => s.id === record.id)) return false;
+		append(LEDGER_FILES.sessions, record);
+		return true;
+	},
+	sessions: () => readAll<SessionRecord>(LEDGER_FILES.sessions),
+
+	appendNode(node: GraphNode) {
+		append(LEDGER_FILES.nodes, node);
+	},
+	nodes: () => readAll<GraphNode>(LEDGER_FILES.nodes),
+
+	appendEdge(edge: GraphEdge) {
+		append(LEDGER_FILES.edges, edge);
+	},
+	edges: () => readAll<GraphEdge>(LEDGER_FILES.edges),
+
+	appendEvent(event: KnowledgeEvent) {
+		append(LEDGER_FILES.events, event);
+	},
+	events: () => readAll<KnowledgeEvent>(LEDGER_FILES.events),
+
+	appendMapping(mapping: SessionMapping) {
+		append(LEDGER_FILES.mappings, mapping);
+	},
+	mappings: () => readAll<SessionMapping>(LEDGER_FILES.mappings),
+};