delete the batch practice-problems module (superseded by practice sessions)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

@@ -4,7 +4,7 @@
  * Split by domain (src/core/contracts/): plan.ts (the session plan tree),
  * graph.ts (the global knowledge graph + its extraction pipeline), chat.ts
  * (session records + UI extension points), jobs.ts (the async sub-agent
- * protocol), practice.ts (the practice-problem system). Each domain file is
+ * protocol),  Each domain file is
  * owned outright by its stream; schema changes to a domain you don't own go
  * through master. This barrel keeps every existing import path
  * ("@/src/core" and "./contracts.ts") unchanged.
@@ -16,4 +16,3 @@ export * from "./contracts/chat.ts";
 export * from "./contracts/jobs.ts";
 export * from "./contracts/probe.ts";
 export * from "./contracts/models.ts";
-export * from "./contracts/practice.ts";