M0 complete: sops-nix wiring + decrypt-a-test-secret; M0 gate CLAIMED

Host decrypts /run/secrets/test_secret via its ssh host key (age identity);
off-box master recovery recipient. sops-nix pinned to a buildGoModule-era rev
for nixpkgs 24.11 compat.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flake.lock

@@ -18,7 +18,29 @@
     },
     "root": {
       "inputs": {
-        "nixpkgs": "nixpkgs"
+        "nixpkgs": "nixpkgs",
+        "sops-nix": "sops-nix"
+      }
+    },
+    "sops-nix": {
+      "inputs": {
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1750119275,
+        "narHash": "sha256-Rr7Pooz9zQbhdVxux16h7URa6mA80Pb/G07T4lHvh0M=",
+        "owner": "Mic92",
+        "repo": "sops-nix",
+        "rev": "77c423a03b9b2b79709ea2cb63336312e78b72e2",
+        "type": "github"
+      },
+      "original": {
+        "owner": "Mic92",
+        "repo": "sops-nix",
+        "rev": "77c423a03b9b2b79709ea2cb63336312e78b72e2",
+        "type": "github"
       }
     }
   },