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

.github/scripts/test-disable-swap.sh

@@ -0,0 +1,37 @@
+#!/usr/bin/env bash
+
+set -Eeuo pipefail
+
+repo_root="$(git rev-parse --show-toplevel)"
+prereq_defaults="$repo_root/roles/prereq/defaults/main.yml"
+prereq_tasks="$repo_root/roles/prereq/tasks/main.yml"
+
+# #670: k3s recommends swap be disabled on all nodes. The prereq role must expose
+# a disable_swap toggle (defaulting to true) that turns swap off now and comments
+# out the /etc/fstab swap entries so swap stays off across reboots.
+grep -Eq -- '^disable_swap: true' "$prereq_defaults" || {
+  printf 'prereq defaults are missing disable_swap: true\n' >&2
+  exit 1
+}
+
+grep -Fq -- 'Disable swap on all cluster nodes' "$prereq_tasks" || {
+  printf 'prereq tasks are missing the swap-disable block\n' >&2
+  exit 1
+}
+
+grep -Fq -- 'swapoff -a' "$prereq_tasks" || {
+  printf 'swap-disable block does not run swapoff -a\n' >&2
+  exit 1
+}
+
+grep -Fq -- '/etc/fstab' "$prereq_tasks" || {
+  printf 'swap-disable block does not comment out /etc/fstab swap entries\n' >&2
+  exit 1
+}
+
+if ! grep -Eq -- 'when: disable_swap' "$prereq_tasks"; then
+  printf 'swap-disable block is not gated on the disable_swap toggle\n' >&2
+  exit 1
+fi
+
+printf 'Swap disable regression test passed\n'