Stop remote info counting a non-blob file under blobs/ (closes #295)

remote info counted every key under blobs/ with four or more path parts
as a blob, while prune skips a name that is not a blob hash. A stray
file such as .DS_Store was reported as an orphaned blob, with advice to
run prune, which then deleted nothing. remote info now takes its blob
list from prune's listAllRemoteBlobs, so its blob and orphan figures,
in the table and in --json, cover exactly what prune would consider.

blobKeyParts moves to prune.go, its only remaining user.

Model: opus-5-5

README.md

@@ -401,7 +401,9 @@ 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_manifest_count`. An unreadable manifest also leaves its
-snapshot's blob count and blob size unknown, `null` in `--json`.
+snapshot's blob count and blob size unknown, `null` in `--json`. A file
+under `blobs/` whose name is not a blob hash is not counted as a blob,
+since `prune` never deletes it.
 * `--json`: Output as JSON
 
 **`remote nuke`**: Delete every snapshot's metadata and every blob from the