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/create_host.py

@@ -314,11 +314,10 @@ def handle_remove(
         for secret_path in host_secrets:
             console.print(f"   [white]vault kv delete secret/{secret_path}[/white]")
 
-    # Warn about secrets directory
+    # Warn about legacy secrets directory
     if secrets_exist:
-        console.print(f"\n[yellow]⚠️  Warning: secrets/{hostname}/ directory exists and will NOT be deleted[/yellow]")
+        console.print(f"\n[yellow]⚠️  Warning: secrets/{hostname}/ directory exists (legacy SOPS)[/yellow]")
         console.print(f"   Manually remove if no longer needed: [white]rm -rf secrets/{hostname}/[/white]")
-        console.print(f"   Also update .sops.yaml to remove the host's age key")
 
     # Exit if dry run
     if dry_run: