feat(secrets): decrypt the login password from sops (task 0010)

Wire sops-nix into the shared base config as unconditional plumbing, with
a two-tier age identity model: an admin identity held outside the repo, and
a per-host identity generated on the machine and kept on its encrypted root.
Both `sshKeyPaths` defaults are cleared so the SSH host keys stay out of the
decryption path and remain free to become secrets in their own right.

The primary user's password hash moves into a shared secrets file encrypted
to admin plus neogaia, consumed through `hashedPasswordFile` and decrypted
before accounts are created.

This needs `users.mutableUsers = false`: NixOS applies a declared hash to an
already-existing account only when that flag is false, so at the default the
hand-set password would have been kept and the change would have been inert.
Root consequently has no password and is locked; sudo from wheel is the way
in, and generation rollback remains the recovery path.

flake.lock

@@ -232,7 +232,28 @@
         "nixpkgs": "nixpkgs_2",
         "nixpkgs-stable": "nixpkgs-stable",
         "nixpkgs-unstable": "nixpkgs-unstable",
-        "nixvim": "nixvim"
+        "nixvim": "nixvim",
+        "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"
       }
     },
     "systems": {