delete the batch practice-problems module (superseded by practice sessions)

Owner ruling 2026-10-04: the module is a sunk cost, retrievable from
history if ever revisited. Removed: core practice contracts + pipeline
helpers, the PracticeStore port + adapter + data dir path, server
service + API routes (sets, clusters picker, practice aborts), the
practice pages + dialog/exam/list UI, chain-table's Generate handoff,
sidebar nav, activity-dock practice rows, and the partitioner/
problemGenerator/grader model roles (BACKGROUND_MODEL_ROLES 8 → 5).

Survivors, moved to core/chains.ts (pure + browser-safe): ReasoningChain,
chainsForSelection, formatChainLine — the chain table and graph
emission still consume them.

app/api/graph/clusters/route.ts

@@ -1,11 +0,0 @@
-/**
- * GET /api/graph/clusters — the birth-session groups for the practice
- * creation picker (owner ruling 2026-09-29: sets are created from graph
- * clusters, node ids only). Thin transport; the grouping is the graph
- * view's own (server/graph.ts → graphSnapshot's clusters).
- */
-import { graphClusters } from "@/src/server";
-
-export async function GET() {
-	return Response.json({ clusters: graphClusters() }, { headers: { "cache-control": "no-store" } });
-}