refactor: rename the three VM hosts to 12kingdoms-*

Drop the now-meaningless kubevirt- prefix and move the hosts out of
hosts/k8s/, following the existing 12kingdoms-shoukei layout: the
directory and output carry the theme prefix while the configuration,
hostname and colmena target are the bare name (shoryu, shushou, youko).

- hosts/k8s/kubevirt-* -> hosts/12kingdoms-*; the shared disko config
  becomes host-disko-fs.nix; the now-different relative imports are fixed
- outputs, vars/networking.nix keys, the Justfile recipes/tags, the eval
  tests and the docs follow
- colmena reaches these hosts by IP (a new targetHost override), since
  their DNS name changes with the hostname and is not resolvable until
  the router learns it

vars/networking.nix

@@ -20,17 +20,17 @@ rec {
 
   hostsAddr = {
     # ============================================
-    # Homelab's Physical Machines (KubeVirt Nodes)
+    # Homelab's Physical Machines (VM hosts)
     # ============================================
-    kubevirt-shoryu = {
+    shoryu = {
       iface = "eno1";
       ipv4 = "192.168.5.181";
     };
-    kubevirt-shushou = {
+    shushou = {
       iface = "eno1";
       ipv4 = "192.168.5.182";
     };
-    kubevirt-youko = {
+    youko = {
       iface = "eno1";
       ipv4 = "192.168.5.183";
     };