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>

scripts/create-host/manipulators.py

@@ -219,7 +219,7 @@ def update_flake_nix(config: HostConfig, repo_root: Path, force: bool = False) -
     new_entry = f"""        {config.hostname} = nixpkgs.lib.nixosSystem {{
           inherit system;
           specialArgs = {{
-            inherit inputs self sops-nix;
+            inherit inputs self;
           }};
           modules = commonModules ++ [
             ./hosts/{config.hostname}