Write only the document to stdout from snapshot remove --json (closes #251)

When the destination store cannot be reached, `snapshot remove` still
removes the snapshot from the local index and warns. The warning went
through the UI, which writes to stdout, so under `--json` it landed
ahead of the document and `| jq` failed on a command that exited 0.
Under `--json` the UI warning is now skipped; the logger's warning on
stderr carries the follow-up, with the snapshot ID as a field.

The follow-up, in the warning, the README and the command's help, said
`vaultik prune` would finish the cleanup, but `prune` never removes
snapshot metadata. They now say to run `vaultik snapshot remove` for the
snapshot again once the destination store is reachable.

Model: opus-5-5

README.md

@@ -352,8 +352,9 @@ may hold snapshots this host doesn't know about), which is what
 prune` invocation to run as a follow-up. Local row cleanup (files,
 chunks, blobs the snapshot was the last referrer for) runs
 automatically. If the destination store is unreachable, the local-DB
-removal still completes and a warning is emitted; rerun `vaultik prune`
-once the store is reachable to finish remote cleanup. To wipe everything
+removal still completes and a warning is emitted; run `vaultik snapshot
+remove <snapshot-id>` again once the store is reachable to remove the
+snapshot's metadata from it (`vaultik prune` does not). To wipe everything
 on the destination in one go, use `vaultik remote nuke --force`.
 * `--local-only`: Skip remote cleanup; only touch the local index
 * `--dry-run`: Show what would be deleted without deleting