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>

hosts/template2/scripts.nix

@@ -2,7 +2,6 @@
 let
   prepare-host-script = pkgs.writeShellApplication {
     name = "prepare-host.sh";
-    runtimeInputs = [ pkgs.age ];
     text = ''
       echo "Removing machine-id"
       rm -f /etc/machine-id || true
@@ -22,11 +21,6 @@ let
 
       echo "Removing cache"
       rm -rf /var/cache/* || true
-
-      echo "Generate age key"
-      rm -rf /var/lib/sops-nix || true
-      mkdir -p /var/lib/sops-nix
-      age-keygen -o /var/lib/sops-nix/key.txt
     '';
   };
 in