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/member_live/form.ex

@@ -226,7 +226,7 @@ defmodule MvWeb.MemberLive.Form do
   def mount(params, _session, socket) do
     # current_user should be set by on_mount hooks (LiveUserAuth + LiveHelpers)
     actor = current_actor(socket)
-    {:ok, custom_fields} = Mv.Membership.list_custom_fields()
+    {:ok, custom_fields} = Mv.Membership.list_custom_fields(actor: actor)
 
     initial_custom_field_values =
       Enum.map(custom_fields, fn cf ->