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

.github/scripts/test-unique-hostname-precheck.sh

@@ -0,0 +1,28 @@
+#!/usr/bin/env bash
+
+set -Eeuo pipefail
+
+repo_root="$(git rev-parse --show-toplevel)"
+site_play="$repo_root/site.yml"
+
+# #636: verify the "Pre tasks" play asserts that all k3s_cluster hosts have
+# unique hostnames, so a duplicate-hostname inventory fails fast instead of
+# silently breaking node registration/joining.
+grep -Fq -- 'Verify all cluster nodes have unique hostnames' "$site_play" || {
+  printf 'site.yml is missing the unique-hostname preflight check\n' >&2
+  exit 1
+}
+
+# The check must deduplicate the cluster hostname list via the `unique` filter
+# and compare lengths, i.e. groups['k3s_cluster'] must be referenced.
+grep -Fq -- "groups['k3s_cluster']" "$site_play" || {
+  printf 'unique-hostname check does not iterate the k3s_cluster group\n' >&2
+  exit 1
+}
+
+if ! grep -Eq -- 'cluster_hostnames.*\|.*unique|\| unique' "$site_play"; then
+  printf 'unique-hostname check does not deduplicate the hostname list\n' >&2
+  exit 1
+fi
+
+printf 'Unique hostname preflight regression test passed\n'