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

Every backup now writes vaultik-destination.txt at the root of the
backup destination store. While the local index holds snapshots, a
destination without it is not present: snapshot create refuses before
writing, prune and snapshot purge fail, and snapshot list and snapshot
remove warn, as for a destination that cannot be listed. remote nuke
deletes the marker last. The file:// backend no longer creates its
destination directory and checks for it on every write.

Tests that backed up into a directory they never created now use an
existing one; fixtures standing for a written destination carry the
marker.

Judgement call: the marker is rewritten on every backup.
Deviation: snapshot name selection moved out of CreateSnapshot unchanged, for the complexity limit.

Model: opus-5-5

internal/vaultik/purge_per_name_test.go

@@ -44,6 +44,9 @@ func setupPurgeTest(
 	repos := database.NewRepositories(db)
 	mockStorage := NewMockStorer()
 
+	// A backup wrote to this destination, so it carries the marker.
+	require.NoError(t, mockStorage.Put(ctx, markerFile, strings.NewReader("")))
+
 	// Insert each snapshot into the DB and create remote metadata stubs.
 	// Use timestamps parsed from snapshot IDs for realistic ordering.
 	for _, id := range snapshotIDs {