feat(prereq): add toggle to disable swap on all cluster nodes (#698)

k3s recommends swap be disabled on every node. Add a disable_swap toggle
(default true) to the prereq role that turns swap off now and comments out the
swap entries in /etc/fstab so swap stays off across reboots. The change is
applied uniformly across all k3s_cluster hosts (all-or-nothing) since leaving
swap on for some nodes but not others creates uneven scheduling and latency.

- roles/prereq/defaults/main.yml: add disable_swap: true default
- roles/prereq/tasks/main.yml: idempotent swapoff -a and fstab comment-out block
  gated on disable_swap
- inventory/sample/group_vars/all.yml: document the new sample variable
- README.md: document the disable_swap option
- .github/scripts/test-disable-swap.sh: regression test
- .pre-commit-config.yaml: wire the test into pre-commit

Closes #670

inventory/sample/group_vars/all.yml

@@ -7,6 +7,10 @@ systemd_dir: /etc/systemd/system
 # Set your timezone
 system_timezone: Your/Timezone
 
+# k3s recommends swap be disabled on every cluster node. Applied uniformly to all
+# nodes (all-or-nothing) in the prereq role. Set to false to leave swap enabled.
+disable_swap: true
+
 # interface which will be used for flannel
 # Defaults to each host's default IPv4 interface (e.g. eth0, enp1s0, ens3)
 # so KVM/cloud hosts without eth0 work out of the box. Override per-host if needed.