Retrieve documents from the knowledge graph with configurable depth and context expansion.

Documents are collected in order:
1. Main document (-k)
2. Children (-d N levels deep)
3. Parent context (-c N levels up)
4. Parents of sub-documents (when -d > 0 and -c > 0)
5. Inline linked documents (-l)

Without -k, --filter, or stdin, the command exits with an error. -k accepts
repeats and resolves to $key (1 key = $eq, 2+ = $in). Use --filter / --included-by
/ --references / etc. to narrow the result set; explicit keys are intersected
with the filter. Empty intersection produces an empty result.
