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/getting-started/quick-start.md

@@ -38,7 +38,7 @@ EOF
 
 # Update Headlamp
 helm upgrade --install headlamp headlamp/headlamp \
-  --namespace kube-system \
+  --namespace headlamp \
   --values headlamp-values.yaml
 ```
 
@@ -70,7 +70,7 @@ metadata:
 subjects:
   - kind: ServiceAccount
     name: headlamp
-    namespace: kube-system
+    namespace: headlamp
 roleRef:
   kind: Role
   name: polaris-proxy-reader
@@ -111,7 +111,7 @@ EOF
 
 ```bash
 # Verify plugin files exist
-kubectl -n kube-system exec -it deployment/headlamp -c headlamp -- \
+kubectl -n headlamp exec -it deployment/headlamp -c headlamp -- \
   ls /headlamp/plugins/headlamp-polaris-plugin/dist/
 
 # Expected output:
@@ -119,7 +119,7 @@ kubectl -n kube-system exec -it deployment/headlamp -c headlamp -- \
 
 # Verify RBAC is correct
 kubectl auth can-i get services/proxy \
-  --as=system:serviceaccount:kube-system:headlamp \
+  --as=system:serviceaccount:headlamp:headlamp \
   -n polaris \
   --resource-name=polaris-dashboard
 
@@ -185,7 +185,7 @@ Cluster score badge in top navigation:
 
 ```bash
 # Verify plugin files exist
-kubectl -n kube-system exec -it deployment/headlamp -c headlamp -- \
+kubectl -n headlamp exec -it deployment/headlamp -c headlamp -- \
   ls /headlamp/plugins/headlamp-polaris-plugin/
 
 # If missing, reinstall via Headlamp UI or sidecar method