The hive Space was created empty — joining it surfaced no rooms because
Matrix doesn't auto-join a Space's children. Provision a default
"hive-chat" room on the matrix sweep, wire it bidirectionally to the
Space (m.space.child on the Space, m.space.parent on the room), and
invite @hive + every agent. The room uses a restricted join rule
allowing any Space member to join, so the operator (a Space member) can
join it from the Space hierarchy without an explicit invite.
Idempotent, mirroring ensure_hive_space: persisted chat-room-id wins,
else rediscover a non-space room named hive-chat, else createRoom. The
space-child link is re-applied each sweep (idempotent PUT) so a
recovered room reconverges its hierarchy link. Room id persisted to
matrix/chat-room-id (0600, survives destroy --purge).
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