delete the batch practice-problems module (superseded by practice sessions)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

@@ -17,6 +17,7 @@
 
 export * from "./contracts.ts";
 export * from "./ports.ts";
+export * from "./chains.ts";
 export * from "./render-settings.ts";
 
 export * from "./question-core.ts";
@@ -35,7 +36,6 @@ export * from "./phases.ts";
 export * from "./pending.ts";
 export * from "./prompt-segments.ts";
 export * from "./math-delims.ts";
-export * from "./practice.ts";
 // Wave B (owner-approved 2026-09-26): the native record (LhMessage), the
 // and the input-events contract — all
 // pure logic, browser-safe.