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/config/config_test.go

@@ -415,7 +415,8 @@ func TestDocumentedSizeExamples(t *testing.T) {
 
 // TestValidatePruneGracePeriod checks that a negative prune_grace_period
 // is refused rather than acting like 0s, which deletes unreferenced blobs
-// at once, and that 0s and the documented examples are accepted.
+// and directories under metadata/ with no manifest at once, and that 0s
+// and the documented examples are accepted.
 func TestValidatePruneGracePeriod(t *testing.T) {
 	t.Parallel()