refactor(nixos): isolate SSH fleet configuration

configs/nixos/modules/network/ssh.nix

@@ -5,70 +5,83 @@
   ...
 }:
 # Configures inbound and outbound SSH access.
-# Keep bindings grouped by purpose rather than dependency traversal.
-# NOLINT(let_order)
 let
-  cfg = config.modules.network.ssh;
-  user = config.user.name;
-
-  workstationNames = [
-    "neogaia"
-    "zeus"
-  ];
-  serverNames = [ "pikachu" ];
-  managedHostNames = workstationNames ++ serverNames;
-  isWorkstation = lib.elem config.networking.hostName workstationNames;
-  isServer = lib.elem config.networking.hostName serverNames;
-
-  clientKeyFor = host: builtins.readFile (../../hosts + "/${host}/ssh_user_ed25519_key.pub");
-  hostKeysFor =
-    host:
-    map builtins.readFile [
-      (../../hosts + "/${host}/ssh_host_ed25519_key.pub")
-      (../../hosts + "/${host}/ssh_host_rsa_key.pub")
-    ];
-  managedTargets = lib.genAttrs managedHostNames (name: {
-    hostName = registry.address.${name}.self;
-    clientKey = clientKeyFor name;
-    hostKeys = hostKeysFor name;
-  });
-  extraTargets = {
-    # TODO: Pin Gitea's host key after migrating it from Proxmox.
-    gitea = {
-      hostName = "git.alexion.dev";
-      port = 2022;
-      user = "gitea";
-      aliases = [
-        "gitea"
-        "git.alexion.dev"
-      ];
-    };
-  };
-  mkTarget = name: target: {
-    hostName = target.hostName or name;
-    user = target.user or user;
-    port = target.port or 22;
-    aliases = target.aliases or [ name ];
-    clientKey = target.clientKey or null;
-    hostKeys = target.hostKeys or [ ];
-  };
-  targets = lib.mapAttrs mkTarget (managedTargets // extraTargets);
-
-  clientKeysFor = names: lib.filter (key: key != null) (map (name: targets.${name}.clientKey) names);
+  cfg = config.modules.network.ssh; # NOLINT(let_order)
   defaultAuthorizedKeys = lib.flatten (
-    clientKeysFor workstationNames ++ lib.optional isServer (clientKeysFor serverNames)
+    clientKeysFor fleet.workstationNames
+    ++ lib.optional fleet.isServer (clientKeysFor fleet.serverNames)
   );
-  outboundTargetNames =
+  clientKeysFor =
+    names: lib.filter (key: key != null) (map (name: fleet.targets.${name}.clientKey) names);
+
+  fleet =
+    # Keep fleet policy and target construction in conceptual order.
+    # NOLINT(let_order)
     let
-      groupTargets =
-        if isServer then
-          [ "gitea" ] ++ serverNames
-        else if isWorkstation then
-          [ "gitea" ] ++ serverNames ++ workstationNames
-        else
-          [ "gitea" ];
+      workstationNames = [
+        "neogaia"
+        "zeus"
+      ];
+      serverNames = [ "pikachu" ];
+      managedHostNames = workstationNames ++ serverNames;
+      isWorkstation = lib.elem config.networking.hostName workstationNames;
+      isServer = lib.elem config.networking.hostName serverNames;
+
+      clientKeyFor = host: builtins.readFile (../../hosts + "/${host}/ssh_user_ed25519_key.pub");
+      hostKeysFor =
+        host:
+        map builtins.readFile [
+          (../../hosts + "/${host}/ssh_host_ed25519_key.pub")
+          (../../hosts + "/${host}/ssh_host_rsa_key.pub")
+        ];
+      managedTargets = lib.genAttrs managedHostNames (name: {
+        hostName = registry.address.${name}.self;
+        clientKey = clientKeyFor name;
+        hostKeys = hostKeysFor name;
+      });
+      extraTargets = {
+        # TODO: Pin Gitea's host key after migrating it from Proxmox.
+        gitea = {
+          hostName = "git.alexion.dev";
+          port = 2022;
+          user = "gitea";
+          aliases = [
+            "gitea"
+            "git.alexion.dev"
+          ];
+        };
+      };
+      mkTarget = name: target: {
+        hostName = target.hostName or name;
+        user = target.user or user;
+        port = target.port or 22;
+        aliases = target.aliases or [ name ];
+        clientKey = target.clientKey or null;
+        hostKeys = target.hostKeys or [ ];
+      };
+      targets = lib.mapAttrs mkTarget (managedTargets // extraTargets);
+      outboundTargetNames =
+        let
+          groupTargets =
+            if isServer then
+              [ "gitea" ] ++ serverNames
+            else if isWorkstation then
+              [ "gitea" ] ++ serverNames ++ workstationNames
+            else
+              [ "gitea" ];
+        in
+        lib.filter (name: name != config.networking.hostName) groupTargets;
     in
-    lib.filter (name: name != config.networking.hostName) groupTargets;
+    {
+      inherit
+        isServer
+        outboundTargetNames
+        serverNames
+        targets
+        workstationNames
+        ;
+    };
+  user = config.user.name;
 
   knownHosts = lib.listToAttrs (
     lib.concatLists (
@@ -81,7 +94,7 @@ let
             publicKey = keyWithoutComment key;
           }
         ) target.hostKeys
-      ) targets
+      ) fleet.targets
     )
   );
   knownHostNamesFor =
@@ -96,7 +109,7 @@ let
   sshSettingsFor =
     name:
     let
-      target = targets.${name};
+      target = fleet.targets.${name};
     in
     {
       header = "Host ${lib.concatStringsSep " " target.aliases}";
@@ -161,7 +174,7 @@ in
           enable = true;
           enableDefaultConfig = false;
           settings = lib.mapAttrs (name: _: sshSettingsFor name) (
-            lib.genAttrs outboundTargetNames (name: name)
+            lib.genAttrs fleet.outboundTargetNames (name: name)
           );
         };
       }