delete the batch practice-problems module (superseded by practice sessions)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

@@ -2,7 +2,7 @@
  * THE NOTIFICATION CHANNEL (push-architecture slice 1): GET /api/events is
  * an SSE response over the core event bus — one connection per open
  * window, held open until the view goes away, fed by every emitter
- * (turn lifecycle, job/chat stores, extraction/practice registries).
+ * (turn lifecycle, job/chat stores, extraction registry).
  *
  * NOTIFY → FETCH (the architecture's whole trick): frames carry only the
  * event type + a tiny payload; consumers respond by fetching truth