Update document content or frontmatter.

Two modes, selected by which flags are present:

  Body-overwrite mode (-k KEY -c CONTENT)
    Rewrites the full markdown body of one document. Use '-' as --content
    to read from stdin.

  Mutation mode (--filter EXPR | -k KEY) + one or more operators
    Applies frontmatter operators (--set / --unset) and block operators
    (--replace, --replace-text, --insert-before, --insert-after, --append,
    --delete) to every document matched by the filter, as one atomic
    update. -k KEY is sugar for '$key: K'; combine with --filter to AND
    further constraints. Each block operator takes '{ <selector>, payload }'
    where the $-keys are the block predicate and the bare keys are the
    payload (content, from/to) plus an optional expect guard. The whole
    update validates before anything is written; on failure it prints the
    offending blocks and exits non-zero.

Body and mutation flags cannot be combined; pass exactly one mode.
