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.

.sops.yaml

@@ -11,7 +11,7 @@ keys:
   # per-host keys — derived from each box's pre-generated SSH host key
   # (ssh-to-age). The host decrypts at runtime using /etc/ssh/ssh_host_ed25519_key.
   - &jupiter age1zak7glavmg4026p2389fyqe769vqm4jrryknuqckgqq4merz5f7q44rkkt
-  - &vps age1hp72xyx2cnd05937e4eww95g5kdtn0wsf9j2nypw330pa69gfdxqn0lpkp
+  - &neptun age1hp72xyx2cnd05937e4eww95g5kdtn0wsf9j2nypw330pa69gfdxqn0lpkp
   # mercury (rpi) uses a dedicated age key (SD image, no ssh-host-key delivery);
   # the private key is dropped on its boot partition after flashing.
   - &mercury age1cpty7zrgnn6l97upq00w5wa8zcvnkxkdt2jvhlj97jh83exure4slha43t
@@ -21,9 +21,9 @@ creation_rules:
   - path_regex: secrets/jupiter\.yaml$
     key_groups:
       - age: [ *admin, *jupiter ]
-  - path_regex: secrets/vps\.yaml$
+  - path_regex: secrets/neptun\.yaml$
     key_groups:
-      - age: [ *admin, *vps ]
+      - age: [ *admin, *neptun ]
   - path_regex: secrets/mercury\.yaml$
     key_groups:
       - age: [ *admin, *mercury ]