A relative index_path resolved against the working directory, so a
backup run from another directory started from an empty index. Validate
now refuses it, naming the value, with the same wording as the file://
storage_url check. ~ is still expanded first. The check sees the final
value, so a relative VAULTIK_INDEX_PATH is refused too; an explicitly
empty index_path is refused as well.
Model: opus-5-5
sneak