delete the batch practice-problems module (superseded by practice sessions)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

@@ -102,7 +102,7 @@ export const askUserQuestionTool: AgentTool<typeof askInputSchema> = {
 // chunk both agents compose (prompts/shared/20-question-tools.md and the
 // teacher's question fragments), reversing the 2026-09-27 placement now
 // that the question family has grown (predict/check roles, future
-// practice variants): duplicates stay honest by sharing SCHEMA ATOMS
+// quiz variants): duplicates stay honest by sharing SCHEMA ATOMS
 // (question-core) + PROMPT CHUNKS, and each tool declares only its
 // mechanical delta.