Delete documents and clean up all references to them.

Targets are selected by either a positional KEY (sugar for '$key: K') or
--filter EXPR (inline YAML filter document, per the spec). Both may be
given; the union is deleted. Reference cleanup applies once over the
whole matched set. Use --dry-run to preview before applying.

Inclusion links to deleted documents are removed entirely; inline links
are converted to plain text.
