delete the batch practice-problems module (superseded by practice sessions)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

@@ -46,9 +46,6 @@ export const BACKGROUND_MODEL_ROLES = [
 	"visualMaker",
 	"extractor",
 	"auditor",
-	"partitioner",
-	"problemGenerator",
-	"grader",
 ] as const;
 
 export type BackgroundModelRole = (typeof BACKGROUND_MODEL_ROLES)[number];
@@ -229,12 +226,6 @@ export const ROLE_SLOT: Readonly<Record<ModelRole, ModelSlotName>> = {
 	auditor: "cheap",
 	visualMaker: "vision",
 	pdfReader: "vision",
-	// The practice trio (2026-09-29): problem engineering is the product's
-	// quality-critical component — strong tier for partitioner/generator/
-	// grader alike (owner-approved plan default; one edit away from cheap).
-	partitioner: "strong",
-	problemGenerator: "strong",
-	grader: "strong",
 };
 
 /** General settings — app-level behavior. Version DISPLAY and plans login