practice slice 3B — the practice agents: partitioner, generator, grader

src/agent/subagents/practice.ts: createPracticeAgents(deps) with the
ExtractionModels mirror (per-stage engine overrides + engineFor +
providerKeyMissing + refuseIfStagesUnready, all three roles strong-tier).
Generator doctrine = the owner's problem-engineering framework verbatim
(minus Sources) + Part III wiring section per the green-lit amendments
(FULL SET input with minimal-use doctrine; degradation never in
meta-commentary). Partitioner: true-partition doctrine, validate via
chainCoverage + exactly-once + unknown-ids, error-back retries through
runSubmitAgent's new validate hook (ToolInputError → error tool result).
Generator: echo law (chains set === target). Grader: verdict union +
nonempty feedback (schema-enforced). renderChains/renderPropositions/
renderParams/renderPreviousProblems. BACKGROUND_MODEL_ROLES extended with
the practice trio (settings override symmetry). runSubmitAgent gains the
validate seam (null = accept). 16-test faux-engine suite incl. retry
paths, refusal gates, neverSubmitted.

tests/settings.test.ts

@@ -202,13 +202,16 @@ describe("sanitizeUserSettings", () => {
 		assert.deepEqual(out.agent?.tools, { quiz: false, demo: true });
 	});
 
-	it("BACKGROUND_MODEL_ROLES matches the five background roles (the labeler died with the wave-7 namer kill)", () => {
+	it("BACKGROUND_MODEL_ROLES matches the eight background roles (practice trio added, owner 2026-09-29)", () => {
 		assert.deepEqual([...BACKGROUND_MODEL_ROLES], [
 			"researcher",
 			"demoBuilder",
 			"visualMaker",
 			"extractor",
 			"auditor",
+			"partitioner",
+			"problemGenerator",
+			"grader",
 		]);
 	});
 });