refactor(nix): move the matrix host options under services.hyperhive.swarm

Second slice of the swarm-global service consolidation, same shape as
the forge move: the operator-facing host options become
services.hyperhive.swarm.matrix.*, and one mkRenamedOptionModule on the
namespace carries the whole subtree (nested gui.* included), so there is
no leaf list to forget an entry from.

The rename lives in hive-matrix.nix, the module that declares the
options, so each service's migration stays independent of its siblings.

The per-agent hyperhive.matrix.{enable,url} and hyperhive.matrixAccounts
are a different namespace -- a client pointer at the service, not the
service -- and deliberately do not move.

nix/host-modules/hyperhive.nix

@@ -36,7 +36,7 @@
     example = "darkest.space";
     description = ''
       Canonical host domain for hyperhive subsystems that need a
-      stable name (currently: `services.hyperhive.matrix.serverName`
+      stable name (currently: `services.hyperhive.swarm.matrix.serverName`
       derives from this, defaulting to
       `matrix.''${services.hyperhive.domain}` when `serverName` is
       null). **Required** when `services.hyperhive.enable` — eval fails