Make remote nuke delete leftover .partial uploads (closes #281)

Every listing skips an object whose name ends in `.partial`, the
temporary name a `file://` or rclone upload writes before moving the
object into place. `remote nuke` deletes only what the listings return,
so it left the `.partial` objects killed uploads leave behind and still
reported the destination store empty.

Storer gains DeletePartialUploads. The file and rclone backends remove
every `.partial` object under the prefix; S3 has none to remove, since
it shows an object only once its upload completes. `remote nuke` calls
it for `metadata/` and `blobs/` as its last step.

Empty directories under a `file://` destination are still left behind.

Model: opus-5-5

TODO.md

@@ -22,6 +22,13 @@ the tag exists and is exercised; what is left is merging `next` to
 
 # Completed Steps
 
+- 2026-10-08: Made `remote nuke` delete the `.partial` files that
+  uploads cut off part-way leave on the destination store
+  ([issue #281](https://git.eeqj.de/sneak/vaultik/issues/281)). Every
+  listing skips such a file, so the command left it in place and still
+  reported the store empty. It now removes them under `metadata/` and
+  `blobs/` as its last step.
+
 - 2026-10-08: Made `remote info` report a snapshot's blob count and
   blob size as unknown when its manifest cannot be read
   ([issue #272](https://git.eeqj.de/sneak/vaultik/issues/272)). The