delete the batch practice-problems module (superseded by practice sessions)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

tests/zero-keys.test.tsx

@@ -128,9 +128,6 @@ const catalog = (strongProvider: string | null, ...keyedProviders: string[]): Mo
 		visualMaker: null,
 		extractor: null,
 		auditor: null,
-		partitioner: null,
-		problemGenerator: null,
-		grader: null,
 	},
 	slots: {
 		strong: strongProvider