Supervises MobAsh.Refresh, the pubsub Registry used to route
post-mutation refresh messages between screens (MOB-56).
Before this module the plugin had no supervision tree; FormScreen
and DetailScreen created / deleted records but never notified
ListScreen to re-read, so the list stayed stale after a pop-back
until the screen was rebuilt from scratch. ListScreen already had a
handle_info(:mob_ash_refresh, ...) clause waiting for a message
nothing was sending.