Add a document extractor tool with an extract_document action that converts an
input artifact (PDF, docx, pptx, ...) into structured JSON by running a fixed
Docling program in the sandbox (the document and parameters travel as data, so
nothing in them can execute). Output is a compact payload bounded by an input
size cap, a head-and-tail markdown window, and per-table caps, can be validated
against a JSON schema, and is persisted as a data artifact by reference. Docling
is heavy, so it stays out of the base image and ships in an opt-in sandbox image
variant.