Make snapshot rm clean up the remote by default

snapshot rm <id> now does the full cleanup: removes the local index
entry, strips the snapshot's metadata from the destination store, and
prunes any blobs that were only referenced by the just-removed manifest.
The --remote flag is retired; --local-only opts out for the rare case
where the user wants to forget a snapshot locally without touching the
remote.

If the destination store is unreachable, the local-DB removal still
completes and a warning is emitted; the user can rerun 'vaultik prune'
to retry the remote half later.

RemoveAllSnapshots gets the same treatment: after deleting every
snapshot's metadata (local + remote + orphan keys), an automatic blob
prune sweep removes the now-unreferenced blob set.

internal/vaultik/prune.go

@@ -27,11 +27,11 @@ func (v *Vaultik) NukeRemote(force bool) error {
 	}
 
 	v.UI.Begin("Removing all snapshot metadata from backup destination store.")
-	if _, err := v.RemoveAllSnapshots(&RemoveOptions{Force: true, Remote: true}); err != nil {
+	if _, err := v.RemoveAllSnapshots(&RemoveOptions{Force: true}); err != nil {
 		return fmt.Errorf("removing all snapshots: %w", err)
 	}
 
-	v.UI.Begin("Removing all blobs from backup destination store.")
+	v.UI.Begin("Removing any blobs still present in backup destination store.")
 	if err := v.PruneBlobs(&PruneOptions{Force: true}); err != nil {
 		return fmt.Errorf("pruning blobs: %w", err)
 	}