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.

config.ts

@@ -53,7 +53,10 @@ export type ModelRole =
 	| "visualMaker"
 	| "pdfReader"
 	| "extractor"
-	| "auditor";
+	| "auditor"
+	| "partitioner"
+	| "problemGenerator"
+	| "grader";
 
 /**
  * The LIVE graph store (src/store/graph.ts) — owned by the app's extraction