Examples:
  iwe tree                           Show full tree with markdown links
  iwe tree -f keys                   Show tree with document keys only
  iwe tree -f json                   Show tree as JSON
  iwe tree -k my-doc                 Show tree starting from 'my-doc'
  iwe tree -k doc-a -k doc-b         Show trees from multiple starting points
  iwe tree --depth 2                 Show only 2 levels deep
  iwe tree --in projects/alpha       Tree rooted at sub-documents of projects/alpha
  iwe tree --in-any a --in-any b     Tree rooted at union of two subtrees
  iwe tree --in a --not-in archive   Sub-documents of a, excluding archive

Structural set selector (--in family):
  --in KEY[:DEPTH]      Sub-documents of EVERY listed key (AND). Repeatable.
  --in-any KEY[:DEPTH]  Sub-documents of AT LEAST ONE listed key (OR).
  --not-in KEY[:DEPTH]  Exclude sub-documents of any listed key (NOT).
  --max-depth N         Default depth for entries without :DEPTH. Unbounded if omitted.

When documents form circular references (A→B→C→A), they have no natural root.
Use -k to start from any document in the cycle:
  iwe tree -k doc-a            View cycle starting from 'doc-a'

Works great with grep to filter the tree:
  iwe tree | grep -i api       Find documents mentioning 'api'
  iwe tree -f keys | grep cli  Find documents with 'cli' in the key
