refactor(nixos): derive managed SSH targets

configs/nixos/modules/network/ssh.nix

@@ -12,7 +12,7 @@ let
   );
   clientKeysFor = names: lib.filter (key: key != null) (map (name: targets.${name}.clientKey) names);
 
-  targets = lib.mapAttrs mkTarget defaultTargets;
+  targets = lib.mapAttrs mkTarget (managedTargets // extraTargets);
   mkTarget = name: target: {
     hostName = target.hostName or name;
     user = target.user or user;
@@ -23,7 +23,26 @@ let
   };
   user = config.user.name;
 
-  defaultTargets = {
+  managedTargets = lib.genAttrs managedHostNames (name: {
+    hostName = registry.address.${name}.self;
+    clientKey = clientKeyFor name;
+    hostKeys = hostKeysFor name;
+  });
+  managedHostNames = workstationNames ++ serverNames;
+  workstationNames = [
+    "neogaia"
+    "zeus"
+  ];
+  serverNames = [ "pikachu" ];
+  clientKeyFor = host: builtins.readFile (../../hosts + "/${host}/ssh_user_ed25519_key.pub");
+  hostKeysFor =
+    host:
+    map builtins.readFile [
+      (../../hosts + "/${host}/ssh_host_ed25519_key.pub")
+      (../../hosts + "/${host}/ssh_host_rsa_key.pub")
+    ];
+
+  extraTargets = {
     # TODO: Pin Gitea's host key after migrating it from Proxmox.
     gitea = {
       hostName = "git.alexion.dev";
@@ -34,38 +53,8 @@ let
         "git.alexion.dev"
       ];
     };
-
-    neogaia = {
-      hostName = registry.address.neogaia.self;
-      clientKey = clientKeyFor "neogaia";
-      hostKeys = hostKeysFor "neogaia";
-    };
-
-    zeus = {
-      hostName = registry.address.zeus.self;
-      clientKey = clientKeyFor "zeus";
-      hostKeys = hostKeysFor "zeus";
-    };
-
-    pikachu = {
-      hostName = registry.address.pikachu.self;
-      clientKey = clientKeyFor "pikachu";
-      hostKeys = hostKeysFor "pikachu";
-    };
   };
-  clientKeyFor = host: builtins.readFile (../../hosts + "/${host}/ssh_user_ed25519_key.pub");
-  hostKeysFor =
-    host:
-    map builtins.readFile [
-      (../../hosts + "/${host}/ssh_host_ed25519_key.pub")
-      (../../hosts + "/${host}/ssh_host_rsa_key.pub")
-    ];
-  workstationNames = [
-    "neogaia"
-    "zeus"
-  ];
   isServer = lib.elem config.networking.hostName serverNames;
-  serverNames = [ "pikachu" ];
 
   knownHosts = lib.listToAttrs (
     lib.concatLists (