HIVE_MATRIX_URL was conditioned on this host deploying the homeserver,
so a hive that is a client of the swarm's matrix forwarded nothing and
every agent's hive-matrix-daemon no-opped. hive-c0re already provisions
those agents' accounts against a remote homeserver through
HIVE_MATRIX_API_URL three lines below, whose gate is `apiUrl != null` —
it registered the users and then never told them where to log in.
gatewayHost is swarm-scoped and defaults to chat.<swarm-domain>, so it
names the swarm's one homeserver from any member hive. Its being set is
the real question, and a null swarm domain is already caught by the
assertion in hive-network.nix rather than by this expression.
e8813b86eb