Shell/core import law: public API barrels + ESLint enforcement (stage 2)

The shell now imports through exactly two barrels, enforced by
import/no-restricted-paths (npm run lint):

  components/**  ->  "@/src/core"      (pure core API, browser-safe)
  app/**         ->  "@/src/core" + "@/src/server" (server entries)

Zone violations are lint errors with architecture-pointer messages; the
pure-core modules (contracts, question-core, plan-tree, phases, pending,
math-delims, ui-message) may never import agent/server/store.

Supporting moves (direction: IO layers -> core):
- core/index.ts: the core public API barrel
- core/ui-message.ts: moved from src/agent (it was shell-facing all along)
- quiz/ask input zod schemas moved agent/tools -> core/question-core
  (the question contract, alongside the grading logic)
- ChatRecord/ChatSummary moved store/chats -> core/contracts (single schema)
- GraphSnapshot/NodeStateClass moved core/graph -> core/contracts (the
  /api/graph payload IS a contract; the pure renderer needs only these)
- src/server/{chats,graph,jobs}.ts + index.ts: thin server entries for the
  transport layer; GET /api/graph's fold side-effect now lives behind
  graphView() instead of inline in the route

Gates: tsc clean, 190/190 tests, eslint clean (law verified to fire on
store/agent/subpath violations), production build clean.

(Repo repair en route: a git object (app/chat/[id] tree) was missing from
the object store — reconstructed byte-exactly via mktree from its blob;
the broken node_modules that stalled the build was cleanly reinstalled.)

src/core/graph.ts

@@ -21,7 +21,16 @@
  */
 
 import { isToolUIPart, type UIMessage } from "ai";
-import type { GraphEdge, GraphNode, LedgerEvent, LedgerEventKind, PlanTree, PlanToolResult } from "./contracts.ts";
+import type {
+	GraphEdge,
+	GraphNode,
+	GraphSnapshot,
+	LedgerEvent,
+	LedgerEventKind,
+	NodeStateClass,
+	PlanTree,
+	PlanToolResult,
+} from "./contracts.ts";
 import type { QuestionAnswerData } from "./question-core.ts";
 import { getChat, listChats } from "../store/chats.ts";
 import { graphLedgers, newGraphNodeId } from "../store/graph.ts";
@@ -308,9 +317,6 @@ export function getGraph(): DerivedGraph {
 	return { nodes, edges: merged, events: graphLedgers.readEvents() };
 }
 
-/** Per-node state classes for the home view (vision §1 "Home page"). */
-export type NodeStateClass = "taught" | "known" | "missed" | "dont-know" | "candidate";
-
 const QUIZ_EVENT_KINDS: ReadonlySet<string> = new Set([
 	"probe-correct",
 	"check-correct",
@@ -359,17 +365,7 @@ export function deriveNodeStates(nodes: GraphNode[], events: LedgerEvent[]): Rec
 
 // ─── the /api/graph payload ──────────────────────────────────────────────────
 
-export interface GraphSnapshot {
-	nodes: GraphNode[];
-	edges: GraphEdge[];
-	nodeStates: Record<string, NodeStateClass>;
-	/** chatId → title, for every chat that touched the graph. */
-	sessions: Record<string, string>;
-	/** nodeId → chatIds that touched the node (click-through provenance). */
-	nodeSessions: Record<string, string[]>;
-}
-
-/** Everything the home graph view needs, in one fetch. */
+/** Everything the home graph view needs, in one fetch (shape: contracts). */
 export function graphSnapshot(): GraphSnapshot {
 	const { nodes, edges, events } = getGraph();
 	const nodeStates = deriveNodeStates(nodes, events);