wip(#2860): c0re-facing matrix.apiUrl option + HIVE_MATRIX_API_URL export

Nix half of the 4th layer mara found (47535: core cannot assume matrix
is on localhost). Rust half (matrix.rs MATRIX_HTTP) NOT done.

Parked here rather than left dirty: she has redirected me to jobq as
prio 1, and uncommitted files migrate across a checkout.

nix/host-modules/hive-c0re/environment.nix

@@ -119,6 +119,20 @@ in
       # at.
       HIVE_MATRIX_URL = "http://${config.services.hyperhive.matrix.gatewayHost}";
     }
+// lib.optionalAttrs (config.services.hyperhive.matrix.apiUrl != null) {
+  # Client-server API base hive-c0re uses to provision matrix (register
+  # agent users, create the hive space + chat room, invite members).
+  # Supplied by `services.hyperhive.matrix.apiUrl`, which the matrix
+  # module fills in with its own loopback listener when it is the thing
+  # running tuwunel — and which the operator sets by hand when the
+  # homeserver lives on another machine.
+  #
+  # NOT the agent-facing HIVE_MATRIX_URL above: that one is the gateway
+  # vhost, and it is absent whenever there is no vhost. Reusing it here
+  # would silently stop provisioning on a hive that runs matrix without
+  # one.
+  HIVE_MATRIX_API_URL = config.services.hyperhive.matrix.apiUrl;
+}
 // lib.optionalAttrs config.services.hyperhive.matrix.gui.enable {
   # Availability flags read by the dashboard's `/api/state`.
   # Matrix GUI lives entirely on the gateway nginx (matrix tab