snapshot rm <id> now does the full cleanup: removes the local index
entry, strips the snapshot's metadata from the destination store, and
prunes any blobs that were only referenced by the just-removed manifest.
The --remote flag is retired; --local-only opts out for the rare case
where the user wants to forget a snapshot locally without touching the
remote.
If the destination store is unreachable, the local-DB removal still
completes and a warning is emitted; the user can rerun 'vaultik prune'
to retry the remote half later.
RemoveAllSnapshots gets the same treatment: after deleting every
snapshot's metadata (local + remote + orphan keys), an automatic blob
prune sweep removes the now-unreferenced blob set.
sneak
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