delete the batch practice-problems module (superseded by practice sessions)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

src/server/stores.ts

@@ -9,7 +9,6 @@
 import { chatStore } from "../store/chats.ts";
 import { graphStore } from "../store/graph.ts";
 import { jobStore } from "../store/jobs.ts";
-import { practiceStore } from "../store/practice.ts";
 import { settingsStore } from "../store/settings.ts";
 import { resolveSettings, type EffectiveSettings } from "../core/settings-resolution.ts";
 /** The wired adapters. Satisfies core/graph.ts's GraphDeps structurally. */
@@ -17,7 +16,6 @@ export const stores = {
 	chatStore,
 	graphStore,
 	jobStore,
-	practiceStore,
 	settingsStore,
 };