deploy: move the matrix homeserver toggle out of swarm

swarm.matrix.enable was the last toggle still spelled swarm.*, which
is the namespace every host in the swarm agrees on - and "does this
host run the homeserver" is exactly what differs between them. It
moves to deploy.matrix.enable with the rest; the genuinely swarm-wide
matrix settings stay where they are.

mkRenamedOptionModule keeps existing configs evaluating with one
warning naming both paths.

The sweep for readers had to be unanchored and cross-language: three
of them were outside nix, including a hive-c0re error message telling
the operator to set the old name. A dotted grep also cannot see the
rename module's own list form, so that was checked separately.

nix/docs/default.nix

@@ -54,7 +54,7 @@ let
           boot.loader.grub.enable = false;
           system.stateVersion = "25.11";
           services.hyperhive.enable = lib.mkForce false;
-          services.hyperhive.swarm.matrix.enable = lib.mkForce false;
+          services.hyperhive.deploy.matrix.enable = lib.mkForce false;
         }
       )
     ];