proxmox: Delete with seed cleanup, health Checker, CRD type comment

Delete stops and destroys a VM of ours under the per-name lock, then
deletes its seed on the VM's node, or on every node when no VM of ours
was left. A hibernated VM of ours is Permanent, as in Create. The
Checker verifies each node concurrently (version, node status, ISO
storage, templates) and adds CertificateInvalid, CredentialsUnavailable,
StorageNotFound and TemplateNotFound to the shared reasons. The
ProviderHealth type comment gains proxmox.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

internal/provider/proxmox/proxmox.go

@@ -172,9 +172,3 @@ func (p *Provider) lockName(ctx context.Context, name string) (func(), error) {
 	}
 	return release, nil
 }
-
-// Delete is a stub until Phase 7. It answers Transient, so a Proxy on this
-// provider retries rather than latching.
-func (p *Provider) Delete(_ context.Context, id string) error {
-	return p.wrapErr("delete", id, transientf("proxmox Delete is not implemented yet"))
-}