sombrero: Add agenix module and use it for pinwheel ssh key

hosts/sombrero/modules/age/default.nix

@@ -0,0 +1,13 @@
+{ inputs, config, ... }:
+let
+  system = config.config-manager.system;
+in
+{
+  age = {
+    identityPaths = [ "/etc/ssh/sombrero" ];
+  };
+
+  environment.systemPackages = [
+    inputs.agenix.packages."${system}".default
+  ];
+}