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

.pre-commit-config.yaml

@@ -68,6 +68,12 @@ repos:
         language: system
         pass_filenames: false
         files: ^site\.yml$|^\.github/scripts/test-unique-hostname-precheck\.sh$
+      - id: disable-swap-test
+        name: Disable swap test
+        entry: .github/scripts/test-disable-swap.sh
+        language: system
+        pass_filenames: false
+        files: ^roles/prereq/(tasks/main|defaults/main)\.yml$|^\.github/scripts/test-disable-swap\.sh$
       - id: cilium-bgp-manifest-test
         name: Cilium BGP manifest test
         entry: python3 .github/scripts/test-cilium-bgp-manifest.py