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

docs/DATAMODEL.md

@@ -230,6 +230,10 @@ The restore process doesn't use the local database. Instead:
    - Query blobs not referenced by any remaining snapshot
    - Delete from S3, once older than `prune_grace_period`, and from the local
      database
+2. **Delete Metadata Directories With No Manifest**
+   - Delete each directory under `metadata/` that has no `manifest.json.zst`
+     and no local snapshot record, once every object in it is older than
+     `prune_grace_period`
 
 ### 6. Incomplete Snapshot Cleanup