An until-green hold is released through the checks service, and a hold set on an already-green head releases at once #6546

Replaces #6529, which was started before the code it covers landed. This waits for #6495 (pull #6508) to land on main.

Done:

• [ ] One test drives a passing check through the checks service's narrowest seam (not by calling releaseOnGreen directly) and sees the until-green hold released with the event hold released: green on <oid>; removing the call in the service makes it fail.
• [ ] Setting an until-green hold on a pull whose current head's latest check is already green releases it at once and says so in the same event; a test covers it.

Files: src/checks/service.ts, src/actions/holds.ts, test/holds.test.ts.

Before pushing: bun x tsc --noEmit and bun test test/holds.test.ts.