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>

README.md

@@ -97,7 +97,7 @@ metadata:
 subjects:
   - kind: ServiceAccount
     name: headlamp # adjust to match your Headlamp service account
-    namespace: kube-system # adjust to match the namespace Headlamp runs in
+    namespace: headlamp # adjust to match the namespace Headlamp runs in
 roleRef:
   kind: Role
   name: polaris-proxy-reader
@@ -197,7 +197,7 @@ npm test
 npm run test:watch
 
 # E2E tests (Playwright)
-export HEADLAMP_TOKEN=$(kubectl create token headlamp -n kube-system --duration=24h)
+export HEADLAMP_TOKEN=$(kubectl create token headlamp -n headlamp --duration=24h)
 npm run e2e
 npm run e2e:headed   # see browser
 ```