delete the batch practice-problems module (superseded by practice sessions)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/core/ports.ts

@@ -21,7 +21,6 @@ import type {
 	ExtractionRunUsage,
 	PlanTree,
 	Position,
-	ProblemSet,
 	CoverageEntry,
 	SessionCheckpoint,
 	TurnUsage,
@@ -157,33 +156,6 @@ export interface JobStore {
 	deleteJobsForChat(chatId: string): number;
 }
 
-// ─── PracticeStore — problem-set documents (data/practice) ──────────────────
-
-/** Everything a new set needs except the store-assigned "ps_" id. */
-export type CreateProblemSetInit = Omit<ProblemSet, "id">;
-
-/**
- * The practice module's persistence: one JSON document per problem set
- * (src/store/practice.ts adapter). Sets are bounded, SELF-CONTAINED
- * snapshots — propositions + chains frozen at creation (contracts/
- * practice.ts) — so reads never join the live graph. Replace-whole
- * writes like settings.json, one-file-per-record like data/jobs.
- * Concurrent same-set writes are the pipeline's serialization duty
- * (slice 4), not the store's — this seam owns file integrity only.
- */
-export interface PracticeStore {
-	/** Persist a new set; the store assigns the id ("ps_" + ts36 + rand). */
-	createSet(init: CreateProblemSetInit): ProblemSet;
-	/** The full set document, or null when unknown id / corrupt file. */
-	getSet(id: string): ProblemSet | null;
-	/** Every set, createdAt descending (the practice page's list view). */
-	listSets(): ProblemSet[];
-	/** Persist the full set document (replace-whole). */
-	saveSet(set: ProblemSet): void;
-	/** Delete a set file; false when unknown id. */
-	deleteSet(id: string): boolean;
-}
-
 // ─── SettingsStore — THE user settings record (settings-storage wave, 2026-09-19) ──
 
 /**