fix(proxmox): auto-select snippets-capable storage for cloud-init

[astrojerms]

Auto-select a snippets-capable storage (Bootstrap QGA robustness follow-up)

The cloud-init snippet path — qemu-guest-agent enablement and the E1 packages/runcmd vendor snippet — required the configured/disk storage to hold snippets. On a local-lvm-only node it silently no-op'd, which stalls DHCP-mode IP discovery for ~10 minutes and drops requested node prep. This is the "real fix" the roadmap's [~] QGA item called for.

Change

• client.PickSnippetsStorage(storages, preferred) — pure, unit-tested: prefer the configured storage when it's snippets-capable, else the node's first snippets-capable storage (sorted for determinism), else ok=false.
• The handler resolves the storage via ListNodeStorages before uploading, so an LVM-default node still lands its snippet on e.g. local.
• Fail-fast: when a VM explicitly requested packages/runcmd and the node has no snippets-capable storage, create returns an actionable error instead of producing a silently-broken node (e.g. Longhorn's open-iscsi never installed). The best-effort agent-only case (no packages/runcmd) still degrades gracefully — logged, non-fatal — preserving prior behavior.

Hardens both the bootstrap QGA path and the just-shipped E1 per-VM vendor snippet (they share applyCloudInitVendorSnippet).

Tests

• PickSnippetsStorage: prefer-configured, fallback-when-preferred-incapable, fallback-when-no-preferred, none-capable.
• Handler: auto-select (LVM default → snippet uploaded to local, cicustom=vendor=local:…) and fail-fast (no snippets storage + packages → error). Existing E1 handler test updated to serve /storage.

Local CI: build, go test ./... (+-race on proxmox), gofmt, vet, staticcheck, golangci-lint (0 issues), modernize — root and the proxmox plugin module — all green.