feat(site): fail fast when cluster nodes share a hostname (#697)

Add a preflight assertion to site.yml that confirms every host in the
k3s_cluster group reports a unique hostname. k3s registers each node keyed by
its hostname, so duplicate hostnames prevent nodes from joining the cluster
and are hard to troubleshoot. The check fails fast in the Pre tasks instead of
surfacing as a cryptic registration failure later.

- site.yml: assert (cluster_hostnames | unique | length) == length for the
  k3s_cluster group, skipping hosts outside that group
- README.md: document the unique-hostname requirement
- .github/scripts/test-unique-hostname-precheck.sh: regression test
- .pre-commit-config.yaml: wire the test into pre-commit

Closes #636

.pre-commit-config.yaml

@@ -62,6 +62,12 @@ repos:
         language: system
         pass_filenames: false
         files: ^roles/k3s_server/tasks/(main|join_master)\.yml$|^\.github/scripts/test-k3s-server-bootstrap\.sh$
+      - id: unique-hostname-precheck-test
+        name: Unique hostname precheck test
+        entry: .github/scripts/test-unique-hostname-precheck.sh
+        language: system
+        pass_filenames: false
+        files: ^site\.yml$|^\.github/scripts/test-unique-hostname-precheck\.sh$
       - id: cilium-bgp-manifest-test
         name: Cilium BGP manifest test
         entry: python3 .github/scripts/test-cilium-bgp-manifest.py