chore(bootstrap): upgrade kubespray to release-2.28 and kubernetes to v1.32.13

- Bump Kubespray baseline to release-2.28 (last release supporting Ubuntu 20.04)
- Upgrade Kubernetes control plane and node to v1.32.13
- Configure single-node cluster settings (drain_nodes: false)
- Add patch for Kubespray v1beta4 kubeadm preflight error handling
- Update init.sh, AGENTS.md, and README.md with two-phase upgrade instructions

bootstrap/patches/kubeadm-upgrade-ignore-preflight.patch

@@ -0,0 +1,20 @@
+diff --git a/roles/kubernetes/control-plane/tasks/kubeadm-upgrade.yml b/roles/kubernetes/control-plane/tasks/kubeadm-upgrade.yml
+index 523956d1d..75723a6e0 100644
+--- a/roles/kubernetes/control-plane/tasks/kubeadm-upgrade.yml
++++ b/roles/kubernetes/control-plane/tasks/kubeadm-upgrade.yml
+@@ -17,6 +17,7 @@
+     --force
+     {%- else %}
+     --config={{ kube_config_dir }}/kubeadm-config.yaml
++    --ignore-preflight-errors={{ kubeadm_ignore_preflight_errors | join(',') }}
+     {%- endif %}
+     {%- if kube_version is version('1.32.0', '>=') %}
+     --skip-phases={{ kubeadm_init_phases_skip | join(',') }}
+@@ -38,6 +39,7 @@
+     {% if kubeadm_patches | length > 0 %}--patches={{ kubeadm_patches_dir }}{% endif %}
+     {%- else %}
+     --config={{ kube_config_dir }}/kubeadm-config.yaml
++    --ignore-preflight-errors={{ kubeadm_ignore_preflight_errors | join(',') }}
+     {%- endif %}
+     --skip-phases={{ kubeadm_upgrade_node_phases_skip | join(',') }}
+   register: kubeadm_upgrade