Unify model changes and shell command argument handling

internal/agents/process.go

@@ -50,11 +50,19 @@ func initialVendorSessionID(vendor api.AgentSessionVendor, id string) string {
 	return id
 }
 
-func checkModel(sess api.AgentSession, model string) error {
-	if vendorOf(model) != sess.Vendor {
-		return fmt.Errorf("%w: session %s is %s; %s is a %s model", ErrInvalid, sess.Id, sess.Vendor, model, vendorOf(model))
+func (m *Manager) changeModel(ctx context.Context, sess api.AgentSession, model, actor string) (api.AgentSession, error) {
+	if model == "" || model == sess.Model {
+		return sess, nil
 	}
-	return nil
+	if vendorOf(model) != sess.Vendor {
+		return api.AgentSession{}, fmt.Errorf("%w: session %s is %s; %s is a %s model", ErrInvalid, sess.Id, sess.Vendor, model, vendorOf(model))
+	}
+	err := m.store.SetSessionModel(ctx, sess.Id, model, actor)
+	if err != nil {
+		return api.AgentSession{}, err
+	}
+	sess.Model = model
+	return sess, nil
 }
 
 type claudeProcess struct {