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

hosts/idols-aquamarine/monitoring/victoriametrics.nix

@@ -4,7 +4,7 @@
   ...
 }:
 let
-  # Powered-off hosts (SBCs, k3s-prod-1) plus shoukei (exporter disabled on the
+  # Powered-off hosts (SBCs) plus shoukei (exporter disabled on the
   # machine); remove entries when they come back online.
   offlineHosts = [
     "shoukei"
@@ -15,12 +15,6 @@ let
     "chiaya"
     "rakushun"
     "mitsuha"
-    "k3s-prod-1-master-1"
-    "k3s-prod-1-master-2"
-    "k3s-prod-1-master-3"
-    "k3s-prod-1-worker-1"
-    "k3s-prod-1-worker-2"
-    "k3s-prod-1-worker-3"
   ];
 in
 {