POST /prompt_manager/images/reveal takes the listing identity
{root_index, root_scope, path} and opens this computer's file manager at
that file. It answers 403 not-local unless the client is loopback, no
proxy header is present and Host/Origin name this computer; the scope is
checked with _gallery_scope_error (409) and the path resolves only inside
that root to a regular media file. Openers run as argv lists with an
absolute executable, no shell and DEVNULL streams (D38).