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

AGENTS.md

@@ -61,7 +61,7 @@ deployments. Keep repository guidance here; reusable global rules live in `agent
 - Deployment and upload recipes change systems; use eval/build commands for validation. Remote
   deployment requires an explicit request. When deployment behavior changes, report the exact `just`
   command to run.
-- KubeVirt hosts (`kubevirt-shoryu`/`shushou`/`youko`): avoid mid-flight restarts of the network
+- KubeVirt hosts (`shoryu`/`shushou`/`youko`): avoid mid-flight restarts of the network
   stack (`systemd-networkd` / the `br0` bridge) — that can drop the VM network. Use `boot` + serial
   reboot for networking or nixpkgs changes. See
   [hosts/README.md](./hosts/README.md#deploying-kubevirt-hosts).