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

roles/prereq/tasks/main.yml

@@ -4,6 +4,28 @@
     name: "{{ system_timezone }}"
   when: (system_timezone is defined) and (system_timezone != "Your/Timezone")
 
+# k3s recommends swap be disabled on all nodes. Disabling swap is all-or-nothing
+# across the cluster: leaving it enabled on some nodes but not others creates
+# uneven scheduling/latency behavior. This block turns swap off and comments out
+# the swap entries in /etc/fstab so it stays off across reboots. It is idempotent
+# and a no-op when swap is already disabled or swapoff is unavailable.
+- name: Disable swap on all cluster nodes
+  when: disable_swap
+  block:
+    - name: Turn off swap now
+      ansible.builtin.command: swapoff -a
+      register: swapoff_result
+      changed_when: false
+      failed_when: false
+
+    - name: Comment out swap entries in fstab
+      ansible.builtin.replace:
+        path: /etc/fstab
+        regexp: '^([^#][^\n]*\s+swap\s+)'
+        replace: '# \\1'
+      register: fstab_swap
+
+
 - name: Set SELinux to disabled state
   ansible.posix.selinux:
     state: disabled