A pure read says the crew's build yield over 24 hours: build runs finished, how many pushed, and how many of those pulls landed

It shows how much crew spend becomes landed work, and it is the first number to look at when the open-pull pile grows.

Done:

[ ] src/flow/build_yield.ts exports readBuildYield(db, now, publicOnly), which returns { finished, pushed, landed, closed } for crew agent runs that finished in the last 24 h. pushed counts runs whose pull got a pull.pushed event (or was opened) between the run's start and end. landed and closed count those pulls that are merged now, or closed without a merge.
[ ] test/flow-build-yield.test.ts seeds a run that pushed and merged, one that pushed and is still open, one that pushed and was closed, and one that never pushed, and checks all four numbers.

Files: src/flow/build_yield.ts, test/flow-build-yield.test.ts

A pure read says the crew's build yield over 24 hours: build runs finished, how many pushed, and how many of those pulls landed #6474