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>

system/default.nix

@@ -10,7 +10,6 @@
     ./nix.nix
     ./root-user.nix
     ./pki/root-ca.nix
-    ./sops.nix
     ./sshd.nix
     ./vault-secrets.nix
   ];

system/sops.nix

@@ -1,7 +0,0 @@
-{ ... }: {
-  sops = {
-    defaultSopsFile = ../secrets/secrets.yaml;
-    age.keyFile = "/var/lib/sops-nix/key.txt";
-    age.generateKey = true;
-  };
-}