Economics ... treats productivity improvements as a mostly exogenous factor produced by black-box engineers and scientists. (If I underestimate your power and merely parody your field, by all means inform me what kind of economic study has been done of such things.) So far as I can tell, economists do not venture into asking where discoveries come from, leaving the mysteries of the brain to cognitive scientists.

Economists do look into the "black box" of where innovations come from. See the fields of "economic growth" and "research policy".

An "insight" is a chunk of knowledge which, if you possess it, decreases the cost of solving a whole range of governed problems.

Yes, but insights vary enormously in how wide a scope of problems they assist. They are probably distributed something like a power law, with many small scope insights, and a few large scope. The large scope insights offer a permanent advantage, but small scope insights remain useful only as long as their scope remains relevant.

Btw, I'm interested in "farming" first because growth rates suddenly increased by two orders of magnitude; by "farming" I mean whatever was the common local-in-time cause of that change. Writing was part of the cascade of changes, but it seems historically implausible to call writing the main cause of the increased growth rate. Professional specialization has more promise as a main cause, but it is still hard to see.