refactor: move host configs under hosts/{jupiter,vps}

- git-mv jupiter/ vps/ into hosts/; fix ../ -> ../../ for common/services/secrets
- flake.nix + deploy point at hosts/<config>/
- README structure updated
- verified: jupiter/vps/vbox all eval

flake.nix

@@ -31,7 +31,7 @@
           modules = [
             disko.nixosModules.disko
             sops-nix.nixosModules.sops
-            ./jupiter/configuration.nix
+            ./hosts/jupiter/configuration.nix
           ];
         };
 
@@ -42,7 +42,7 @@
           modules = [
             disko.nixosModules.disko
             sops-nix.nixosModules.sops
-            ./vps/configuration.nix
+            ./hosts/vps/configuration.nix
           ];
         };
 
@@ -52,7 +52,7 @@
         jupiter-vbox = nixpkgs.lib.nixosSystem {
           inherit system;
           specialArgs = { inherit inputs; };
-          modules = [ ./jupiter/vm.nix ];
+          modules = [ ./hosts/jupiter/vm.nix ];
         };
 
         # Custom kexec installer with our SSH key baked in, for headless install