Returns the normalized brief, the fields the brief did not state, matched suppliers with
match reasons and source evidence, the filters applied with record counts, and paging
fields (offset, limit, has_more). A brief with no recognisable process, material, finish,
or location returns no records and a follow-up question instead of the whole index.
`query` is 3 to 5000 characters, `limit` 1 to 20, `offset` 0 to 1000. When you already
know the buyer's material, process, finish, or location, pass them as the optional
fields: each overrides the parsed value (for example material="aluminum 6061",
process="CNC turning", finish="anodizing", location="Dongguan"); a value the index cannot
match is ignored and listed in `retrieval.ignored_fields`.