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.)

package.json

@@ -9,6 +9,7 @@
 		"build": "next build",
 		"start": "next start",
 		"test": "NODE_ENV=test tsx --test tests/*.test.ts tests/*.test.tsx",
+		"lint": "eslint .",
 		"typecheck": "tsc --noEmit"
 	},
 	"dependencies": {
@@ -54,11 +55,16 @@
 		"zod": "^4.4.3"
 	},
 	"devDependencies": {
+		"@next/eslint-plugin-next": "^16.3.3",
 		"@tailwindcss/postcss": "^4.3.3",
 		"@types/node": "^24.0.0",
 		"@types/react": "^19.2.18",
 		"@types/react-dom": "^19.2.4",
 		"@types/turndown": "^5.0.6",
+		"@typescript-eslint/parser": "^8.68.0",
+		"eslint": "^9.39.5",
+		"eslint-import-resolver-typescript": "^4.4.5",
+		"eslint-plugin-import": "^2.32.0",
 		"tailwindcss": "^4.3.3",
 		"tsx": "^4.19.0",
 		"typescript": "^5.6.0"