Revert CLAUDE.md, README.md, and SECURITY.md namespace references from
'headlamp' back to 'kube-system' to match src/api/k8s.ts KUBE_VIP_NAMESPACE.
The source code uses 'kube-system' as the namespace constant; docs must
match code to avoid misleading users.
Also fixes hardcoded namespace string in OverviewPage.tsx to use
KUBE_VIP_NAMESPACE constant instead of literal 'kube-system'.
Option A per PRI-562 QA guidance.
Co-Authored-By: Paperclip <noreply@paperclip.ing>