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`.

flake.lock

@@ -264,7 +264,28 @@
         "nixos-generators": "nixos-generators",
         "nixos-hardware": "nixos-hardware",
         "nixpkgs": "nixpkgs_3",
-        "nixpkgs-unstable": "nixpkgs-unstable"
+        "nixpkgs-unstable": "nixpkgs-unstable",
+        "sops-nix": "sops-nix"
+      }
+    },
+    "sops-nix": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1788337237,
+        "narHash": "sha256-gkSH8VUtCo6hnysNmb9DbTuDepH2t5pv+QWjP75xKAk=",
+        "owner": "Mic92",
+        "repo": "sops-nix",
+        "rev": "fbf759290e0cb0a98dfc813a4eb7d53ad1dacb57",
+        "type": "github"
+      },
+      "original": {
+        "owner": "Mic92",
+        "repo": "sops-nix",
+        "type": "github"
       }
     }
   },