Make remote nuke delete every object under metadata/ and blobs/ (closes #291)

remote nuke deleted only the snapshot metadata and blobs the snapshot
and blob listings recognise, so a snapshot's database whose manifest
was never uploaded, and any name under blobs/ that is not a blob hash,
survived while nuke reported the store empty.

After removing every snapshot, nuke now deletes every object listed
under metadata/ and blobs/, whatever its name, in place of the blob
prune step. A delete that fails is logged and the others still run. A
listing afterwards decides the result: nuke reports the store empty
only when it finds nothing, and otherwise names each object left and
fails.

Judgement call: nuke no longer prints the blob prune summary.

Model: opus-5-5

README.md

@@ -413,11 +413,12 @@ 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
-backup destination store, leaving the bucket prefix empty. Destructive and
-irreversible. This is the single supported way to wipe the entire
-destination store. If anything cannot be deleted, the command exits
-non-zero; running it again deletes what is left.
+**`remote nuke`**: Delete every object under `metadata/` and `blobs/` on
+the backup destination store, whatever its name, leaving the bucket prefix
+empty. Destructive and irreversible. This is the single supported way to
+wipe the entire destination store. If a listing afterwards still finds
+anything there, the command names each object left and exits non-zero;
+running it again deletes what is left.
 * `--force`: Required to confirm destruction.
 
 **`database delete`**: Delete the local SQLite state database file