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.)

components/chat.tsx

@@ -20,11 +20,8 @@ import { Composer } from "@/components/composer";
 import { QuestionDock } from "@/components/question-dock";
 import { SidebarToggle } from "@/components/sidebar";
 import { TreePanel } from "@/components/tree-panel";
-import type { TeacherUIMessage } from "@/src/agent/ui-message";
-import type { ArtifactPartProps } from "@/src/core/contracts";
-import type { QuestionAnswerData } from "@/src/core/question-core";
-import { findPendingJobCall, findPendingToolCall } from "@/src/core/pending";
-import { phaseAt } from "@/src/core/phases";
+import type { ArtifactPartProps, QuestionAnswerData, TeacherUIMessage } from "@/src/core";
+import { findPendingJobCall, findPendingToolCall, phaseAt } from "@/src/core";
 
 /** One width for everything: transcript, cards, and the bottom blob share this column. */
 const COLUMN = "mx-auto w-full max-w-3xl px-4";
@@ -101,7 +98,7 @@ export function Chat({
 		) {
 			void sendMessage();
 		}
-		// eslint-disable-next-line react-hooks/exhaustive-deps -- mount-only by design
+		// mount-only by design
 	}, []);
 
 	const pending = findPendingToolCall(messages);
@@ -318,7 +315,7 @@ function DockOverlay({
 			setExpanded(isAtBottom);
 			if (isAtBottom) void scrollToBottom();
 		}
-		// eslint-disable-next-line react-hooks/exhaustive-deps -- fire only when a NEW question lands
+		// fire only when a NEW question lands
 	}, [pendingId]);
 
 	// ── Collapse state machine with hysteresis ──