nix: hyperhive.westonRdp.enable — opt-in weston + RDP backend per agent

nix/templates/harness-base.nix

@@ -10,6 +10,11 @@
   # this. The systemd service that actually runs the harness binary
   # differs per role and lives in the child module.
 
+  # Optional feature modules. Each declares its own `hyperhive.*`
+  # option(s), default-off, so every agent has them available but
+  # only opts in from its own `agent.nix`.
+  imports = [ ./weston-rdp.nix ];
+
   options.hyperhive.allowedRecipients = lib.mkOption {
     type = lib.types.listOf lib.types.str;
     default = [ ];