practice slice 3A — runSubmitAgent extracted to shared module; practice model roles

runSubmitAgent moved verbatim from extraction.ts to src/agent/subagents/
submit-agent.ts (exported; extraction imports it — zero behavior change,
extraction suites green untouched). New optional prepareArguments seam:
extraction wires its healStringifiedArrays heal; default is identity
pass-through. ModelRole += partitioner | problemGenerator | grader;
ROLE_SLOT pins all three to strong (quality-critical trio, plan default).
Note: BACKGROUND_MODEL_ROLES (per-role settings overrides) deliberately
NOT extended — settings-surface decision, flagged for the planning
session.

src/core/settings.ts

@@ -226,6 +226,12 @@ export const ROLE_SLOT: Readonly<Record<ModelRole, ModelSlotName>> = {
 	auditor: "cheap",
 	visualMaker: "vision",
 	pdfReader: "vision",
+	// The practice trio (2026-09-29): problem engineering is the product's
+	// quality-critical component — strong tier for partitioner/generator/
+	// grader alike (owner-approved plan default; one edit away from cheap).
+	partitioner: "strong",
+	problemGenerator: "strong",
+	grader: "strong",
 };
 
 /** General settings — app-level behavior. Version DISPLAY and plans login