delete the batch practice-problems module (superseded by practice sessions)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

@@ -6,7 +6,7 @@ import { usePathname, useRouter } from "next/navigation";
 import { MenuShell } from "@/components/menu-shell";
 import { DeleteSessionDialog } from "@/components/delete-session-dialog";
 import type { ChatDeleteMode } from "@/src/core";
-import { GraduationCapIcon, PanelLeftCloseIcon, PanelLeftOpenIcon, PlusIcon, SearchIcon, SettingsIcon } from "lucide-react";
+import { PanelLeftCloseIcon, PanelLeftOpenIcon, PlusIcon, SearchIcon, SettingsIcon } from "lucide-react";
 import { ActivitySection } from "@/components/activity-dock";
 import { CoverageMenuBlock } from "@/components/extraction-coverage";
 import { LhMark } from "@/components/lh-mark";
@@ -655,21 +655,6 @@ function SessionsSidebar() {
 					<span className="sr-only">create a new session</span>
 				</button>
 
-				{/* Practice (slice 5, 2026-09-29): the set list's entry — the same
-				    quiet row language as New session, active-state lit while any
-				    /practice route is open. GraduationCapIcon to match the dock. */}
-				<button
-					type="button"
-					data-testid="nav-practice"
-					onClick={() => router.push("/practice")}
-					className={`mx-3 mb-1 flex items-center gap-1.5 rounded-md px-2 py-1.5 text-xs uppercase tracking-wider transition-colors hover:bg-accent hover:text-foreground ${
-						pathname.startsWith("/practice") ? "bg-accent text-foreground" : "text-muted-foreground"
-					}`}
-				>
-					<GraduationCapIcon className="size-3" />
-					Practice
-				</button>
-
 				<div className="mx-3 mb-1 flex items-center gap-2">
 					<span className="text-xs uppercase tracking-widest text-muted-foreground/70">Sessions</span>
 					<Hint label="Search sessions" action="sessions-search" side="bottom">