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/host-modules/deploy.nix

@@ -152,6 +152,24 @@ in
       '';
     };
 
+    matrix.enable = lib.mkOption {
+      type = lib.types.bool;
+      default = false;
+      description = ''
+        Run the swarm's matrix homeserver — matrix-tuwunel, in a
+        `hive-matrix` container — on this host.
+
+        Derives from
+        {option}`services.hyperhive.swarm.enableRequiredServices` with
+        the other once-per-swarm services. Set it here directly to put
+        the homeserver somewhere other than the host holding the rest.
+
+        Client-side settings stay in
+        {option}`services.hyperhive.swarm.matrix.*`, which every hive
+        agrees on; this is only the decision to run it here.
+      '';
+    };
+
     nats.enable = lib.mkOption {
       type = lib.types.bool;
       default = false;