refactor: rename vps host -> neptun (solar-system theme)

git-mv hosts/vps->neptun, secrets/vps.yaml->neptun.yaml; update flake, .sops.yaml
rules, hostName, sops paths, README/CLAUDE. Off-repo host key dir renamed too.

hosts/neptun/hardware-configuration.nix

@@ -0,0 +1,23 @@
+# PLACEHOLDER — replace on install.
+#
+# disko (disk-config.nix) owns the filesystems, so this file only carries
+# kernel modules + platform. nixos-anywhere regenerates it via:
+#   nixos-generate-config --no-filesystems
+# Keep the imports/kernel-module lines; the virtio modules are pinned in
+# configuration.nix so root mounts even if the generator misses them.
+{ config, lib, pkgs, modulesPath, ... }:
+
+{
+  imports = [ (modulesPath + "/profiles/qemu-guest.nix") ];
+
+  boot.initrd.availableKernelModules = [ "ahci" "xhci_pci" "virtio_pci" "virtio_blk" "sr_mod" ];
+  boot.initrd.kernelModules = [ ];
+  boot.kernelModules = [ ];
+  boot.extraModulePackages = [ ];
+
+  # NO fileSystems here — disko defines "/" and "/boot".
+
+  swapDevices = [ ];
+
+  nixpkgs.hostPlatform = lib.mkDefault "x86_64-linux";
+}