Document System Actor pattern in code guidelines

Add section explaining when and how to use system actor for systemic operations.
Include examples and distinction between user mode and system mode.

test/mv/helpers/system_actor_test.exs

@@ -79,7 +79,10 @@ defmodule Mv.Helpers.SystemActorTest do
           |> Ash.load!(:role, domain: Mv.Accounts)
 
         _ ->
-          Accounts.create_user!(%{email: admin_email}, upsert?: true, upsert_identity: :unique_email)
+          Accounts.create_user!(%{email: admin_email},
+            upsert?: true,
+            upsert_identity: :unique_email
+          )
           |> Ash.Changeset.for_update(:update, %{})
           |> Ash.Changeset.manage_relationship(:role, admin_role, type: :append_and_remove)
           |> Ash.update!()
@@ -152,6 +155,7 @@ defmodule Mv.Helpers.SystemActorTest do
       end
 
       admin_email = System.get_env("ADMIN_EMAIL") || "admin@localhost"
+
       case Accounts.User
            |> Ash.Query.filter(email == ^admin_email)
            |> Ash.read_one(domain: Mv.Accounts) do