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

remote info counted every four-part key under blobs/ 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

@@ -393,7 +393,9 @@ 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. A name under `metadata/` that is not a remote
-key is skipped with a warning and is not printed. If a listed
+key is skipped with a warning and is not printed. A file under `blobs/`
+whose name is not a blob hash is not counted as a blob, since `prune`
+never deletes it. 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