OUTPUT FORMATS:

MARKDOWN (default, -f markdown):
  Each document has YAML frontmatter followed by content:

    ---
    document:
      key: doc-a
      title: Document A
      parents:           # omitted if empty
      - key: parent-doc
        title: Parent Doc
      back-links:        # omitted if empty
      - key: other-doc
        title: Other Doc
    ---

    # Document A

    Content here...

  Multiple documents are separated by frontmatter delimiters (---).
  Two empty lines follow each document.

  Sub-documents show other parents with <- notation:

    # Child Doc

    [Child Doc](child-doc) <- [Other Parent](other-parent)

  This indicates child-doc is also embedded in other-parent, helping
  you understand where else a sub-document is used.

KEYS (-f keys):
  One document key per line, suitable for piping:

    doc-a
    child-doc
    parent-doc

JSON (-f json):
  Structured output:

    {"documents": [{"key": "...", "title": "...",
      "content": "...", "parent_documents": [...],
      "child_documents": [...], "backlinks": [...]}]}