Delete metadata directories a backup left without a manifest (closes #390)

A backup that stopped between uploading db.zst.age and its manifest
left that directory under metadata/ on the destination for good:
prune deleted nothing there.

The blob step of prune, which snapshot create --prune also runs, now
deletes each directory under metadata/ that has no manifest.json.zst,
that no local snapshot record claims, and all of whose objects are
older than prune_grace_period; an object listed with no time keeps its
directory, and only names that are remote keys are deleted. The prompt
and the --force output name each such directory, the --json result
counts them (metadata_dirs_deleted, metadata_dirs_failed), and a failed
delete fails the run. Every description of the grace period now
covers these directories.

Model: opus-5-5

internal/cli/prune.go

@@ -13,8 +13,9 @@ func NewPruneCommand() *cobra.Command {
 	cmd := &cobra.Command{
 		Use:   "prune",
 		Short: "Tidy local database and remote storage",
-		Long: `Removes orphaned data from both the local index database and
-unreferenced blobs from the backup destination store.
+		Long: `Removes orphaned data from the local index database, and
+unreferenced blobs and directories under metadata/ with no manifest
+from the backup destination store.
 
 Local cleanup drops incomplete snapshots and any files, chunks, or
 blobs no longer referenced by a completed snapshot. Remote cleanup
@@ -26,6 +27,10 @@ another sharing the destination store, that has not yet uploaded its
 manifest. The grace period does not protect a blob such a backup
 reuses from an earlier run that stopped before its manifest: that
 blob keeps the time of its first upload, so it can be deleted.
+Remote cleanup also deletes each directory under metadata/ with no
+manifest, as a backup that stopped before its manifest leaves, once
+all its objects are older than prune_grace_period and its snapshot is
+not in the local index.
 
 Snapshot create --prune runs the same cleanup automatically; this
 command is the manual entry point for the same work (e.g. after a