[noels]

Part of GIT-18. Moves #30's microk8s-hostpath default-class override so it's public and exists iff Longhorn is enabled.

Unflagging the distro's default StorageClass is a MicroK8s + Longhorn concern: every MicroK8s cluster running Longhorn has two defaults. It isn't an ivylen fact. So it's keyed on the cluster's distro label, not on the cluster.

• Appset: a third source (after the chart, so the private overlay's sources/1 value-file patches are unaffected; verified with the gitops-private bootstrap build). It syncs environments/default/addons/longhorn/distros/{{metadata.labels.distro}} with the longhorn Application.
• distros/microk8s/: the add-on's microk8s-hostpath object copied field-for-field, with is-default-class: "false" and Prune=false,Delete=false (PVCs still use it by name). Server-side diff against ivylen: none; the live object already matches after GIT-18: single default StorageClass on ivylen (Longhorn) #30.
• distros/kind/: resources: []. Every distro label in use needs a folder, or the app fails with "path does not exist"; distros/README.md explains this. Labels today: microk8s (on-prem), kind (hub/spokes).
• clusters/ivylen/resources/ is back to []. The live object has Prune=false,Delete=false, so leaving that list neither deletes nor reverts it. The longhorn app adopts it once enable_longhorn is applied.
• Drops the example file added earlier in this PR (replaced by the real mechanism). Supersedes gitops-private#14 (closed).

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