Two claims this branch falsified and left standing, both caught in
review:
`vhosts.nix`'s `errorPages` param comment enumerated the set
(`{ notFound, unreachable, unauthorized }`) and adding a fourth member
made the enumeration wrong at the point a reader consults it.
`gateway.md` said extending custom error pages beyond the per-agent
routes was "a separate follow-up" — while this branch is that follow-up,
so the doc contradicted the code sitting next to it.
Rewrites the scope rule as the criterion rather than a list, since a
list is what went stale: a route earns a page when the default status
would point at the wrong component. That covers the per-agent routes and
the sso vhost, and explains why forge/matrix/fluffychat still don't
qualify — their upstreams being down means what the code says.
ac15c68cd2