feat: sops-nix for samba password secret

- add sops-nix input + module (jupiter only, not the VM)
- secrets/jupiter.yaml: age-encrypted samba_password (safe to commit)
- .sops.yaml: encryption rule for admin age key
- secrets.nix: decrypt samba_password to /run/secrets on the host
- provisioning oneshot reads sops secret (host) or plaintext (VM), single value
- .sops private key stays off-repo (~/.config, /var/lib/sops-nix on host)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

flake.lock

@@ -39,7 +39,28 @@
     "root": {
       "inputs": {
         "disko": "disko",
-        "nixpkgs": "nixpkgs"
+        "nixpkgs": "nixpkgs",
+        "sops-nix": "sops-nix"
+      }
+    },
+    "sops-nix": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1783174389,
+        "narHash": "sha256-aCWC8ngycU7OdJrU2+Je3qf+1a2ykuBvpPhZT/9tXMc=",
+        "owner": "Mic92",
+        "repo": "sops-nix",
+        "rev": "f1406619a3884cd5c47992a70b8b35c9c0fcb4c9",
+        "type": "github"
+      },
+      "original": {
+        "owner": "Mic92",
+        "repo": "sops-nix",
+        "type": "github"
       }
     }
   },