Step 1 of the hive-gateway.nix vhost-builder cleanup. The two
sub-domain vhosts (forge, matrix) were inline `lib.optionalAttrs`
blocks `//`-appended to the virtualHosts attrset, burying the
top-level structure. Lift them into `forgeVhost` / `matrixVhost`
bindings in the config-closure let so the composition reads as
`{ "_" = ...; } // forgeVhost // matrixVhost` — the three vhosts are
now visible at a glance. Also fixes a garbled merge-mangled comment on
the nginx-reload host-trigger.
Pure readability refactor, eval-identical: verified the generated
`services.nginx.virtualHosts` toJSON is byte-identical before/after
(8888 bytes, diff empty) on a host with matrix+forge+gui+auth+tls all
enabled. The deeper `_` vhost location-group extraction is a follow-up.
a03aafb004