feat(k8s): Longhorn Platform component (F1)

[astrojerms]

F1 — Longhorn Platform component

Adds longhorn to the opt-in Platform composite: replicated block storage (RWO volumes with node-to-node replication + snapshots) — the storage tier NFS/local-path don't cover.

apiVersion: k8s.openctl.io/v1
kind: Platform
metadata: { name: home }
spec:
  kubeconfigPath: { $ref: {apiVersion: k3s.openctl.io/v1, kind: Cluster, name: home, field: status.outputs.kubeconfigPath} }
  longhorn:
    enabled: true

• Chart longhorn from https://charts.longhorn.io, default namespace longhorn-system.
• #Platform.longhorn? schema entry, mirroring the other components.
• Unit test: chart/namespace defaults + values thread-through.

Node prerequisite

Longhorn needs open-iscsi on every node. Install it via a k3s worker pool's nodePrep.packages: [open-iscsi] (E2, just shipped) — called out in both the component comment and the schema doc — or Longhorn's own iscsi-installer DaemonSet as an interim fallback. Opt-in like every Platform component (nothing enabled by default).

Local CI (k8s plugin module): build, go test ./..., gofmt, vet, staticcheck, golangci-lint (0 issues), modernize — all green.