chore: remove the never-deployed k3s-prod-1 cluster (#284)

It was never bootstrapped (no flux deploy key, VMs commented out since
2024-10) and only carried a stale duplicate of the test cluster's config.

- hosts/k8s/k3s-prod-1-{master,worker}-{1,2,3} + flake outputs + the home module
- the k3s-prod-1-token agenix entry (the .age file lives in the private secrets repo)
- the k3s-prod-1-* IPs, the offlineHosts entries, the Justfile recipes and the docs

secrets/nixos.nix

@@ -200,11 +200,6 @@ in
 
     (mkIf cfg.server.kubernetes.enable {
       age.secrets = {
-        "k3s-prod-1-token" = {
-          file = "${mysecrets}/server/k3s-prod-1-token.age";
-        }
-        // high_security;
-
         "k3s-test-1-token" = {
           file = "${mysecrets}/server/k3s-test-1-token.age";
         }