delete the batch practice-problems module (superseded by practice sessions)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

@@ -1,6 +1,6 @@
 /**
  * runSubmitAgent — the shared single-submit agent seam (moved verbatim from
- * extraction.ts, practice slice 3, 2026-09-29): one TypeBox-validated submit
+ * extraction.ts): one TypeBox-validated submit
  * call is the whole deliverable (a pi agent purely so the model can retry a
  * schema-invalid call). The submitted value is captured from the execute
  * seam, which only fires on a schema-valid call — the ??-fill (the old zod