feat(matrix): auto-create a hive chat room as a child of the hive Space

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).

hive-c0re/src/paths.rs

@@ -81,6 +81,13 @@ pub fn matrix_space_room_id() -> PathBuf {
     matrix_dir().join("space-room-id")
 }
 
+/// `matrix/chat-room-id` — persisted default "hive chat" room id (the
+/// `m.space.child` of the hive Space every agent + the operator can join).
+#[must_use]
+pub fn matrix_chat_room_id() -> PathBuf {
+    matrix_dir().join("chat-room-id")
+}
+
 /// `matrix/creds/` — per-agent throwaway matrix passwords (survive
 /// `destroy --purge`; agents auth by token, this is recovery only).
 #[must_use]