docs: update Headlamp install namespace references from kube-system to headlamp

Updates all documentation references to the Headlamp install namespace
from kube-system to headlamp as part of PRI-433.

In-scope files updated:
- README.md, SECURITY.md
- docs/getting-started/installation.md, quick-start.md, prerequisites.md
- docs/deployment/helm.md, kubernetes.md, production.md
- docs/troubleshooting/README.md, common-issues.md, rbac-issues.md
- docs/user-guide/configuration.md, rbac-permissions.md
- docs/TESTING.md, TROUBLESHOOTING.md, DEPLOYMENT.md

Out-of-scope (unchanged):
- Source files referencing upstream workload namespace
- RBAC manifests describing Polaris namespace (polaris ns is unchanged)
- NetworkPolicy namespaceSelector (API server runs in kube-system)
- design-decisions.md and ARCHITECTURE.md (URL hashes refer to cluster namespaces, not Headlamp install ns)

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

docs/DEPLOYMENT.md

@@ -33,7 +33,7 @@ kubectl -n polaris get svc polaris-dashboard
 kubectl get --raw /api/v1/namespaces/polaris/services/polaris-dashboard:80/proxy/results.json | jq .PolarisOutputVersion
 
 # Verify Headlamp is deployed
-kubectl -n kube-system get pods -l app.kubernetes.io/name=headlamp
+kubectl -n headlamp get pods -l app.kubernetes.io/name=headlamp
 ```
 
 ## Installation Methods
@@ -59,7 +59,7 @@ kubectl -n kube-system get pods -l app.kubernetes.io/name=headlamp
 
    ```bash
    helm upgrade --install headlamp headlamp/headlamp \
-     --namespace kube-system \
+     --namespace headlamp \
      --values headlamp-values.yaml
    ```