ci: rework E2E infrastructure to use default namespace

Board directive: E2E tests must run in the `default` namespace.
Nothing should persist beyond a test run; no dedicated namespace needed.

Changes:
- e2e-ci-runner-rbac.yaml: retarget Role/RoleBinding to `default`,
  remove ClusterRole/ClusterRoleBinding (no longer needed since we
  don't need cluster-scoped namespace read permission)
- e2e.yaml: set E2E_NAMESPACE=default
- deploy-e2e-headlamp.sh: default namespace to `default`, remove
  namespace existence check (default always exists)
- teardown-e2e-headlamp.sh: default namespace to `default`, remove
  namespace existence check guard
- headlamp-e2e-values.yaml: update usage comment
- e2e/README.md: remove namespace creation prerequisite

Closes #78 #79

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

scripts/teardown-e2e-headlamp.sh

@@ -4,21 +4,15 @@
 # Tears down the dedicated E2E Headlamp instance deployed by deploy-e2e-headlamp.sh.
 #
 # Environment:
-#   E2E_NAMESPACE  — namespace to clean up (default: headlamp-e2e)
+#   E2E_NAMESPACE  — namespace to clean up (default: default)
 #   E2E_RELEASE    — Helm release to uninstall (default: headlamp-e2e)
 set -euo pipefail
 
 REPO_ROOT="$(cd "$(dirname "$0")/.." && pwd)"
 
-E2E_NAMESPACE="${E2E_NAMESPACE:-headlamp-e2e}"
+E2E_NAMESPACE="${E2E_NAMESPACE:-default}"
 E2E_RELEASE="${E2E_RELEASE:-headlamp-e2e}"
 
-# Exit early if the namespace does not exist — nothing to tear down.
-if ! kubectl get namespace "$E2E_NAMESPACE" >/dev/null 2>&1; then
-  echo "Namespace $E2E_NAMESPACE does not exist, nothing to tear down."
-  exit 0
-fi
-
 echo "=== E2E Headlamp Teardown ==="
 echo "  Namespace: $E2E_NAMESPACE"
 echo "  Release:   $E2E_RELEASE"
@@ -32,9 +26,6 @@ kubectl delete configmap headlamp-polaris-plugin -n "$E2E_NAMESPACE" --ignore-no
 echo "Cleaning up service account..."
 kubectl delete serviceaccount headlamp-e2e-test -n "$E2E_NAMESPACE" --ignore-not-found
 
-# Note: namespace is NOT deleted — it is managed by a cluster admin.
-# The runner SA only has namespace-scoped permissions (see deployment/e2e-ci-runner-rbac.yaml).
-
 # Clean up local env file
 rm -f "$REPO_ROOT/.env.e2e"