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 a remote key, or an unambiguous leading
part of it, with the same resolveSnapshotRemoteKey that verify and
restore use. The prompt, the dry run and the warning after a failed
remote delete name the snapshot it resolved to: its ID when known,
otherwise its full remote key. Under --json, snapshot_id holds the ID
only when it is known, and remote_key holds the full key.

Model: deepseek-v4-pro (change); opus-5-5 (rebase onto next, rework)

README.md

@@ -448,6 +448,8 @@ 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