refactor: redesign E2E to use custom Docker image instead of PVC/kubectl

Replace the PVC + kubectl-patch approach for E2E plugin deployment with
a custom Docker image that has the plugin pre-installed. This eliminates
all policy-violating operations:

- No PVCs in kube-system
- No kubectl exec/cp to Headlamp pods
- No deployment patching via kubectl
- No temporary pods or ConfigMap-based file transfers

The new approach builds a Headlamp image with the plugin baked in
(Dockerfile.e2e), deploys it as a dedicated instance in the headlamp-e2e
namespace via Helm, and tears it down after tests complete.

RBAC is scoped to the headlamp-e2e namespace instead of kube-system.

Note: .github/workflows/e2e.yaml still needs updating to use the new
scripts — that change is delegated to Hugh (CI/CD owner).

Closes: privilegedescalation/headlamp-polaris-plugin#72

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

.gitignore

@@ -6,5 +6,6 @@ e2e/.auth/
 test-results/
 .playwright-mcp/
 .env
+.env.e2e
 .env.local
 .eslintcache