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

internal/vaultik/prune.go

@@ -23,6 +23,10 @@ var errNukeRequiresForce = errors.New(
 // metadata.
 const metadataDirName = "metadata"
 
+// blobKeyParts is the "/"-separated segment count of a blob object key
+// (blobs/<aa>/<bb>/<hash>).
+const blobKeyParts = 4
+
 // NukeRemote deletes every snapshot's metadata and every blob from remote
 // storage, along with any object an upload cut off part-way left under a
 // temporary `.partial` name. After this returns successfully the bucket