fix: CustomField policies, no system-actor fallback, guidelines

- Tests and UI pass actor for CustomField create/read/destroy; seeds use actor
- Member required-custom-fields validation uses context.actor only (no fallback)
- CODE_GUIDELINES: add rule forbidding system-actor fallbacks

lib/mv_web/live/global_settings_live.ex

@@ -130,6 +130,7 @@ defmodule MvWeb.GlobalSettingsLive do
           :if={@active_editing_section != :member_fields}
           module={MvWeb.CustomFieldLive.IndexComponent}
           id="custom-fields-component"
+          actor={@current_user}
         />
       </.form_section>