should_announce_auto_update_default() -> boolReturn whether to show the one-time auto-update default migration notice.
True when no explicit env/config preference is set (so auto-update is on
only implicitly, via the opt-out default) and the notice has not been
acknowledged yet. This does not itself verify that auto-update is enabled;
callers must gate on is_auto_update_enabled first (e.g. an editable
install has no explicit preference but never auto-updates).