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

internal/cli/snapshot.go

@@ -288,6 +288,10 @@ know about), which is what 'vaultik prune' does. The command prints the
 'vaultik prune' invocation to run as a follow-up only when it removed the
 snapshot's metadata from the destination store, and never under --json.
 
+The snapshot may be named by its ID or, on a host with no local index, by
+the remote key that 'snapshot list' prints for a remote-only snapshot (an
+unambiguous leading part is enough).
+
 An ID that names no snapshot in the local index or on the destination
 store is an error.