Refuse a backup destination an unplugged volume left empty (closes #239)

Every backup writes vaultik-destination.txt at the destination root.
While the local index holds snapshots stored there, a destination
without it is not present: snapshot create refuses before writing,
prune and snapshot purge fail, and snapshot list and snapshot remove
act as for a destination that cannot be listed. snapshot create and
snapshot remove check it first. remote nuke deletes the marker last.
The file:// backend never creates its destination directory, which
protects a directory on a removable volume, not a mount point.

Tests back up into directories that exist; fixtures for a written
destination carry the marker.

Judgement call: the marker is rewritten on every backup.
Deviation: for the complexity limit, two blocks moved, unchanged, into
snapshotNamesToCreate and confirmRemoveSnapshot.

Model: opus-5-5

internal/vaultik/prune.go

@@ -42,9 +42,10 @@ const blobKeyParts = 4
 
 // NukeRemote deletes every object under metadata/ and blobs/ in remote
 // storage, whatever its name, along with any object an upload cut off
-// part-way left under a temporary `.partial` name. It returns
-// successfully only when a listing afterwards finds nothing left there,
-// and the next backup then starts from scratch.
+// part-way left under a temporary `.partial` name, and last the
+// destination marker. It returns successfully only when a listing
+// afterwards finds nothing left under metadata/ and blobs/, and the next
+// backup then starts from scratch.
 //
 // Refuses to run unless force is true. The caller is responsible for
 // confirming with the user.
@@ -105,6 +106,13 @@ func (v *Vaultik) emptyDestinationStore() error {
 		return fmt.Errorf("%d %w", len(left), errObjectsLeft)
 	}
 
+	// Last, once the local snapshot records are gone: the next backup
+	// then has none and writes the marker again.
+	err = v.Storage.Delete(v.ctx, destinationMarkerKey)
+	if err != nil {
+		return fmt.Errorf("deleting %s: %w", destinationMarkerKey, err)
+	}
+
 	return nil
 }