Fix model fallback across main/background/sync agents

src/shared/session-model-state.ts

@@ -0,0 +1,15 @@
+export type SessionModel = { providerID: string; modelID: string }
+
+const sessionModels = new Map<string, SessionModel>()
+
+export function setSessionModel(sessionID: string, model: SessionModel): void {
+  sessionModels.set(sessionID, model)
+}
+
+export function getSessionModel(sessionID: string): SessionModel | undefined {
+  return sessionModels.get(sessionID)
+}
+
+export function clearSessionModel(sessionID: string): void {
+  sessionModels.delete(sessionID)
+}