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-body.test.ts

@@ -49,7 +49,7 @@ describe("parseAgentSettingsBody", () => {
 		});
 		const bad = parseAgentSettingsBody({ models: { teacher: {} } });
 		assert.equal(bad.ok, false);
-		if (!bad.ok) assert.match(bad.error, /unknown role "teacher" \(supported: researcher, demoBuilder, visualMaker, extractor, auditor\)/);
+		if (!bad.ok) assert.match(bad.error, /unknown role "teacher" \(supported: researcher, demoBuilder, visualMaker, extractor, auditor, partitioner, problemGenerator, grader\)/);
 		const malformed = parseAgentSettingsBody({ models: { researcher: { model: { provider: 1, id: "x" } } } });
 		assert.equal(malformed.ok, false);
 	});