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-e2e-values.yaml

@@ -0,0 +1,22 @@
+---
+# Headlamp Helm values for E2E testing with shared volume plugin deployment.
+#
+# The CI runner and Headlamp pod share a PVC so that the runner can copy
+# built plugin artifacts directly into Headlamp's plugins directory.
+# This is a CI-only mechanism — production plugin distribution uses ArtifactHub.
+
+# Point Headlamp at the shared plugins mount
+config:
+  pluginsDir: /headlamp/plugins
+
+# PVC-backed volume shared with the CI runner
+volumes:
+  - name: plugins
+    persistentVolumeClaim:
+      claimName: headlamp-plugins
+
+# Mount into the Headlamp container
+volumeMounts:
+  - name: plugins
+    mountPath: /headlamp/plugins
+    readOnly: true