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/snapshot_failed_file_test.go

@@ -87,7 +87,7 @@ func TestSnapshotSummaryCountsFileNotStoredAsFailed(t *testing.T) {
 				"src": {Paths: []string{srcDir}},
 			}
 
-			store, err := storage.NewFileStorer(filepath.Join(tempDir, "remote"))
+			store, err := storage.NewFileStorer(t.TempDir())
 			require.NoError(t, err)
 
 			db, err := database.New(ctx, cfg.IndexPath)