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, --in, or stdin, the command exits with an error. When --in / --in-any /
--not-in / --max-depth are provided, the selector resolves to a set of keys to retrieve.
Combining -k with the selector intersects the two — explicit keys filtered through the
selector. Empty intersection produces an empty result.
