e2e: shared volume plugin deployment replacing init container approach

Replace the init container plugin installation with a shared PVC volume
between the CI runner and Headlamp pod. The runner builds the plugin and
copies it to the shared mount; Headlamp reads from the same volume.

- Add deployment/headlamp-e2e-values.yaml (PVC-backed shared volume)
- Add deployment/headlamp-plugins-pvc.yaml (PVC manifest)
- Add scripts/deploy-plugin-via-volume.sh (build + copy + restart)
- Remove deployment/headlamp-static-plugin-values.yaml (init container)

This is CI-only test infrastructure — ArtifactHub remains the sole
user-facing distribution channel.

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

deployment/headlamp-plugins-pvc.yaml

@@ -0,0 +1,14 @@
+---
+# PVC for sharing built plugin artifacts between the CI runner and Headlamp.
+# Used only in E2E test environments — not for production.
+apiVersion: v1
+kind: PersistentVolumeClaim
+metadata:
+  name: headlamp-plugins
+  namespace: kube-system
+spec:
+  accessModes:
+    - ReadWriteOnce
+  resources:
+    requests:
+      storage: 128Mi