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/configuration.nix

@@ -18,7 +18,7 @@
   # what nixos-generate-config detects in the installer.
   boot.initrd.availableKernelModules = [ "virtio_pci" "virtio_blk" "virtio_scsi" ];
 
-  networking.hostName = "vps";
+  networking.hostName = "neptun";
 
   # ---- Static networking (netcup) ----
   # No LAN fallback: get this right or the box is unreachable (use netcup's

hosts/neptun/secrets.nix

@@ -1,11 +1,11 @@
 { config, ... }:
 
-# sops-nix wiring for the VPS. Encrypted values live in ../../secrets/vps.yaml,
+# sops-nix wiring for neptun (netcup VPS). Encrypted values in ../../secrets/neptun.yaml,
 # decrypted with the VPS's own SSH host key (recipient in ../../.sops.yaml).
 # The host key is pre-generated on the laptop and shipped at install
 # (nixos-anywhere --extra-files -> /etc/ssh/ssh_host_ed25519_key).
 {
-  sops.defaultSopsFile = ../../secrets/vps.yaml;
+  sops.defaultSopsFile = ../../secrets/neptun.yaml;
   sops.age.sshKeyPaths = [ "/etc/ssh/ssh_host_ed25519_key" ];
 
   sops.secrets.tailscale_authkey = { };