ci(runner): restore isolated Molecule CI

- Scope VirtualBox cleanup to repository-owned Molecule resources\n- Route lint to hosted runners and harden self-hosted workflow execution\n- Add diagnostics, fixture coverage, and reproducible lint validation

.github/scripts/collect-runner-diagnostics.sh

@@ -0,0 +1,36 @@
+#!/usr/bin/env bash
+
+set -Eeuo pipefail
+
+output_dir="${1:-${RUNNER_TEMP:-/tmp}/k3s-ci-diagnostics}"
+mkdir -p -- "$output_dir"
+umask 077
+
+run_capture() {
+  local output_file="$1"
+  shift
+  {
+    printf '$'
+    printf ' %q' "$@"
+    printf '\n'
+    "$@"
+  } > "$output_dir/$output_file" 2>&1 || true
+}
+
+run_capture system.txt uname -a
+run_capture runner-user.txt id
+run_capture memory.txt free -h
+run_capture disk.txt df -h
+run_capture virtualbox-version VBoxManage --version
+run_capture virtualbox-vms VBoxManage list vms
+run_capture virtualbox-running-vms VBoxManage list runningvms
+run_capture virtualbox-disks VBoxManage list hdds
+run_capture virtualbox-hostonlyifs VBoxManage list hostonlyifs
+run_capture vagrant-status vagrant global-status
+run_capture molecule-state find "${HOME}/.cache/molecule" -maxdepth 6 -type f -path '*/.vagrant/machines/*/virtualbox/id' -print
+
+if [[ -r /etc/vbox/networks.conf ]]; then
+  cp -- /etc/vbox/networks.conf "$output_dir/virtualbox-networks.conf"
+fi
+
+printf 'Diagnostics written to %s\n' "$output_dir"