feat(sops): add sops-nix input, core module and devShell tooling

Wires sops-nix into every machine via machines/core/sops.nix so hosts
decrypt with their existing SSH host key. Adds a devShell exposing
sops/age/ssh-to-age for the operator workflow, which also fixes .envrc's
previously no-op `use flake`.

machines/configuration.nix

@@ -4,6 +4,7 @@ let
     nixpkgs
     nixpkgs-unstable
     nixos-hardware
+    sops-nix
     #hyprland
     ;
   nixosSystem = nixpkgs.lib.makeOverridable nixpkgs.lib.nixosSystem;
@@ -40,6 +41,7 @@ let
       ];
     }
     ../modules
+    sops-nix.nixosModules.sops
     # ../profiles
   ];
   defaultModules = baseModules ++ customModules;