flake: remove sops-nix (no longer used)

All secrets are now managed by OpenBao (Vault). Remove the legacy
sops-nix infrastructure that is no longer in use.

Removed:
- sops-nix flake input
- system/sops.nix module
- .sops.yaml configuration file
- Age key generation from template prepare-host scripts

Updated:
- flake.nix - removed sops-nix references from all hosts
- flake.lock - removed sops-nix input
- scripts/create-host/ - removed sops references
- CLAUDE.md - removed SOPS documentation

Note: secrets/ directory should be manually removed by the user.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

flake.lock

@@ -100,28 +100,7 @@
         "homelab-deploy": "homelab-deploy",
         "nixos-exporter": "nixos-exporter",
         "nixpkgs": "nixpkgs",
-        "nixpkgs-unstable": "nixpkgs-unstable",
-        "sops-nix": "sops-nix"
-      }
-    },
-    "sops-nix": {
-      "inputs": {
-        "nixpkgs": [
-          "nixpkgs-unstable"
-        ]
-      },
-      "locked": {
-        "lastModified": 1770145881,
-        "narHash": "sha256-ktjWTq+D5MTXQcL9N6cDZXUf9kX8JBLLBLT0ZyOTSYY=",
-        "owner": "Mic92",
-        "repo": "sops-nix",
-        "rev": "17eea6f3816ba6568b8c81db8a4e6ca438b30b7c",
-        "type": "github"
-      },
-      "original": {
-        "owner": "Mic92",
-        "repo": "sops-nix",
-        "type": "github"
+        "nixpkgs-unstable": "nixpkgs-unstable"
       }
     }
   },