delete the batch practice-problems module (superseded by practice sessions)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

tests/settings.test.ts

@@ -203,16 +203,13 @@ describe("sanitizeUserSettings", () => {
 		assert.deepEqual(out.agent?.tools, { quiz: false, demo: true });
 	});
 
-	it("BACKGROUND_MODEL_ROLES matches the eight background roles (practice trio 2026-09-29)", () => {
+	it("BACKGROUND_MODEL_ROLES matches the five background roles (practice trio deleted 2026-10-04)", () => {
 		assert.deepEqual([...BACKGROUND_MODEL_ROLES], [
 			"researcher",
 			"demoBuilder",
 			"visualMaker",
 			"extractor",
 			"auditor",
-			"partitioner",
-			"problemGenerator",
-			"grader",
 		]);
 	});
 });