Wave 0: shared contracts + extension points

- src/core/contracts.ts: plan tree / graph / ledger / job protocol / artifact
  registry types — the single source of truth for all workstreams
- tools: per-request factory createTeacherTools({chatId}) composing
  questionTools + planTools (stub) + jobTools (stub); quiz gains nodeId
- prompt composition: system-prompt.md + sorted src/agent/prompts/*.md
- ChatRecord.tree + saveTree; pending gate scoped to quiz/ask (+findPendingJobCall)
- chat.tsx: artifact renderer extension point + ChatActionsContext.syncFromServer

src/store/chats.ts

@@ -2,6 +2,7 @@ import * as fs from "node:fs";
 import * as path from "node:path";
 import type { UIMessage } from "ai";
 import { PATHS } from "../../config.ts";
+import type { PlanTree } from "../core/contracts.ts";
 
 /**
  * Chat store — one JSON file per chat in data/chats/.
@@ -19,6 +20,8 @@ export interface ChatRecord {
 	createdAt: string;
 	updatedAt: string;
 	messages: UIMessage[];
+	/** The session's plan tree (vision §2) — written by plan tools via saveTree. */
+	tree?: PlanTree;
 }
 
 export interface ChatSummary {
@@ -80,6 +83,16 @@ export function saveMessages(id: string, messages: UIMessage[]): void {
 	fs.writeFileSync(chatFile(id), JSON.stringify(record, null, "\t"), "utf-8");
 }
 
+/** Persist the session plan tree (plan tools call this in execute). */
+export function saveTree(id: string, tree: PlanTree): void {
+	const record = getChat(id);
+	if (!record) throw new Error(`Chat not found: ${id}`);
+	record.tree = tree;
+	// Deliberately does NOT bump updatedAt: tree mutations piggyback on the
+	// turn's saveMessages, and sidebar ordering shouldn't churn on tree edits.
+	fs.writeFileSync(chatFile(id), JSON.stringify(record, null, "\t"), "utf-8");
+}
+
 /** Derive a short title from the first user message. */
 export function titleFromPrompt(prompt: string): string {
 	const oneLine = prompt.replace(/\s+/g, " ").trim();