snapshot remove accepts a remote-only snapshot's remote key (closes #400)

A host with no local index could remove a remote-only snapshot only by
its plain snapshot ID, which it does not have. snapshot remove passed
its argument through RemoteSnapshotKey, rehashing a remote key so it
never matched. It now resolves the identifier with the same
resolveSnapshotRemoteKey that verify and restore use, so a remote key
or an unambiguous leading part of it works.

Model: deepseek-v4-pro

README.md

@@ -446,6 +446,9 @@ once the store is reachable to remove the snapshot's metadata from it
 (`vaultik prune` does not). For a snapshot that is only on the
 destination store, that failure is an error. To wipe everything on the
 destination in one go, use `vaultik remote nuke --force`.
+* Accepts a snapshot ID, or — for a snapshot only on the destination
+  store — its remote key (or an unambiguous leading part of it) as shown
+  by `snapshot list`.
 * `--local-only`: Skip remote cleanup; only touch the local index
 * `--dry-run`: Show what would be deleted without deleting
 * `--force`: Skip confirmation prompt