docs: redirect install namespace references from kube-system to headlamp

Update documentation to reference the Headlamp plugin namespace (headlamp)
instead of kube-system where kube-vip itself is deployed.

Files changed (all docs only):
- README.md: requirements, troubleshooting table
- CLAUDE.md: data sources, key constants namespace
- SECURITY.md: plugin scope permissions list

Out of scope — left untouched per PRI-340 plan:
- Source files (k8s.ts, KubeVipDataContext.tsx, OverviewPage.tsx)
- Test helpers (test-helpers.tsx) — kube-system is the watched workload namespace
- ADR 003 — describes kube-vip static pod fallback behavior, not install namespace

Co-Authored-By: Paperclip <noreply@paperclip.ing>

SECURITY.md

@@ -12,10 +12,10 @@ This plugin is **read-only**. It does not perform any write operations against t
 
 - Services (type: LoadBalancer)
 - Nodes
-- Pods in `kube-system`
-- DaemonSets in `kube-system`
-- Leases in `kube-system`
-- ConfigMaps in `kube-system`
+- Pods in `headlamp`
+- DaemonSets in `headlamp`
+- Leases in `headlamp`
+- ConfigMaps in `headlamp`
 
 All data is fetched through Headlamp's built-in API proxy, which respects the user's existing RBAC permissions.