docs(matrix): document hive Matrix Space creation and persistence

docs/matrix.md

@@ -122,6 +122,25 @@ Initial rollout settings:
 - `allow_encryption = false` per operator call; E2EE re-enabling is
   deferred to a follow-up.
 
+## Hive Matrix Space
+
+On first boot (after all agent accounts are provisioned), hive-c0re
+creates a private **Matrix Space** named `"hive"` using the admin
+account (`@hive:<server_name>`) and invites every provisioned agent
+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.
+
+**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).
+
 ## Assertion rationale
 
 Two `config.assertions` entries fail eval early rather than ship