delete the batch practice-problems module (superseded by practice sessions)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

@@ -20,7 +20,6 @@
  */
 
 import { extractionStatuses, type ExtractionPhase } from "../agent/subagents/extraction.ts";
-import { practiceStatuses, type PracticePhase } from "../agent/subagents/practice-pipeline.ts";
 import { chatStore } from "../store/chats.ts";
 import { graphRevision } from "./graph.ts";
 import { catalogContextLimits } from "./model-catalog.ts";
@@ -28,7 +27,6 @@ import { listJobs } from "./jobs.ts";
 import type { JobRecord } from "../core/contracts.ts";
 
 export { abortExtraction, extractionStatuses, type ExtractionPhase, type RunningExtraction } from "../agent/subagents/extraction.ts";
-export { practiceStatuses, type PracticePhase, type RunningPractice } from "../agent/subagents/practice-pipeline.ts";
 export { chatStore } from "../store/chats.ts";
 
 /** A running sub-agent job, joined with its chat's title for the dock.
@@ -56,21 +54,6 @@ export interface ActivityExtraction {
 	startedAt: number;
 }
 
-/** A live practice ROW in the dock's feed — per-problem, matching the
- *  extraction per-chunk pattern (owner ruling, 2026-09-29). id is the
- *  ROW KEY (`${setId}:partition` | `${setId}:gen:${pid}` |
- *  `${setId}:grade:${pid}`) — the dock's abort target; problemIndex/
- *  problemTotal ride when the row belongs to a fan-out. */
-export interface ActivityPractice {
-	id: string;
-	kind: "generation" | "grading";
-	title: string;
-	phase: PracticePhase;
-	startedAt: number;
-	problemIndex?: number;
-	problemTotal?: number;
-}
-
 /** The dock's whole world in one payload: every running process, every view.
  *  Jobs are RUNNING-ONLY — finished jobs belong to their chat's transcript;
  *  the blob disappears when the process ends. Cost math is NOT done here:
@@ -85,7 +68,6 @@ export interface ActivityPractice {
 export function activityFeed(): {
 	jobs: ActivityJob[];
 	extractions: ActivityExtraction[];
-	practice: ActivityPractice[];
 	graphRevision: number;
 } {
 	const chats = chatStore.listChats();
@@ -115,21 +97,11 @@ export function activityFeed(): {
 		phase: run.phase,
 		startedAt: run.startedAt,
 	}));
-	const practice: ActivityPractice[] = practiceStatuses().map((row) => ({
-		id: row.key,
-		kind: row.kind,
-		title: row.label,
-		phase: row.phase,
-		startedAt: row.startedAt,
-		problemIndex: row.problemIndex,
-		problemTotal: row.problemTotal,
-	}));
 	// oldest-first on last activity: the stable blob order (ISO strings sort chronologically)
 	jobs.sort((a, b) => a.updatedAt.localeCompare(b.updatedAt));
 	return {
 		jobs,
 		extractions: extractions.sort((a, b) => a.startedAt - b.startedAt),
-		practice: practice.sort((a, b) => a.startedAt - b.startedAt),
 		graphRevision: graphRevision(),
 	};
 }