delete the batch practice-problems module (superseded by practice sessions)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

components/setup-dialog.tsx

@@ -49,7 +49,7 @@ import type { ThinkingLevel } from "@earendil-works/pi-agent-core";
 /** The pickers' slot identity — same roster language as the Agent
  *  section's slot rows. */
 const SLOT_META: Record<ModelSlotName, { label: string; roster: string }> = {
-	strong: { label: "Strong", roster: "Prober, teacher, practice trio" },
+	strong: { label: "Strong", roster: "Prober, teacher" },
 	cheap: { label: "Cheap", roster: "Researcher, demo builder, extractor, auditor" },
 	vision: { label: "Vision", roster: "Visual maker + PDF reader" },
 };