Leave remote info orphan figures unknown when a manifest is unreadable (closes #228)

When a manifest could not be read, remote info skipped it, counted that
snapshot's blobs as orphaned and advised running prune. The orphan
figures are now unknown in that case, with no prune advice; --json gives
them as null and lists the unreadable remote keys in
unreadable_manifests. Only a listed manifest.json.zst is read, so a
directory without one, as an interrupted backup leaves, keeps the
figures known.

Names under metadata/ were used unchecked and printed raw. A name that
is not a remote key is now skipped with a warning. A manifest under it
is not read either, so it also leaves the figures unknown; --json counts
such manifests in skipped_manifests.

The closing log line carries the unreadable manifest count in place of
the orphan count.

Model: opus-5-5

README.md

@@ -390,7 +390,13 @@ recipients, and local database statistics.
 
 **`remote info`**: Show storage backend type and location plus detailed
 remote storage inventory: per-snapshot metadata sizes, blob counts, and
-orphaned blob detection.
+orphaned blob detection. A name under `metadata/` that is not a remote
+key is skipped with a warning and is not printed. If a listed
+`manifest.json.zst` cannot be read, or sits under a skipped name, the
+orphaned blob figures are reported as unknown; `--json` gives them as
+`null`, lists the remote key of each unreadable manifest in
+`unreadable_manifests` and counts the manifests under skipped names in
+`skipped_manifests`.
 * `--json`: Output as JSON
 
 **`remote nuke`**: Delete every snapshot's metadata and every blob from the