Display document hierarchy as a tree structure.

By default, shows root documents (entry points with no incoming block references)
and their children organized in a tree format. Each level of nesting represents
a block reference relationship between documents.

Use -k/--key to start the tree from specific document(s), regardless of whether
they are roots. -k accepts repeats: 1 key uses $eq semantics, 2+ uses $in.

Combine with --filter / --included-by / --references / etc. to narrow the
roots further (intersection). Empty intersection yields an empty tree.

Output formats:
  markdown  Nested list with links (default): - [Title](key)
  keys      Document keys only
  json      JSON array with nested structure
  yaml      YAML rendering of the same structure

Use -d/--depth to limit how deep to traverse (default: 4).
