`format_for_agent` named the database whenever one was named, so a search over
a single named database carried a line with nothing to distinguish. It now takes
`include_collection` from the caller, which decides from the search selection
rather than from the hits: a search that could have drawn on two collections
names them even when everything came back from one.
`Collection:` at the model boundary, database in configuration and
administration. `source` on results, documents, citations and analysis
dictionaries is unchanged.
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