docs: document hive-chat room auto-creation in Matrix Space (follow-up to #1452)

docs/matrix.md

@@ -131,15 +131,26 @@ into it. This gives the operator a single Space in FluffyChat or any
 Matrix client that groups all agent-to-agent + operator rooms in one
 place.
 
-**State**: the room ID is persisted to
-`/var/lib/hyperhive/matrix/space-room-id` (mode `0600`, root-owned).
-This path is **outside** every agent state dir and is NOT deleted by
-`nixos-container destroy --purge` — the Space survives full agent
-purges and is reused on re-provision.
+The sweep also provisions a default **`hive-chat` room** as an
+`m.space.child` of the Space. Joining a Space doesn't auto-join
+child rooms — the explicit room entry ensures the operator and every
+agent can find a common chat room without manual setup. Room join is
+restricted (any Space member including the operator can join; agents
+are explicitly invited). Room version pinned to 10 for the restricted
+join floor.
 
-**Idempotent**: if the file exists and is non-empty, the Space is
-considered already created. Delete the file to force re-creation (e.g.
-after a homeserver wipe).
+**State**: both room IDs are persisted to `/var/lib/hyperhive/matrix/`
+(mode `0600`, root-owned):
+- `space-room-id` — the Space itself
+- `chat-room-id` — the `hive-chat` room
+
+These paths are **outside** every agent state dir and are NOT deleted by
+`nixos-container destroy --purge` — both survive full agent purges and
+are reused on re-provision.
+
+**Idempotent**: if the files exist and are non-empty, the Space and
+room are considered already created. Delete the files to force
+re-creation (e.g. after a homeserver wipe).
 
 ## Configuration tuning