The planner can close an issue as split when every issue it was split into has already closed as done #2515

Why: #477, #2454 and #2285 were split and every piece landed (#2323, #2442, #2443, #2492, #2493, #2494). The planner still cannot close them. handedToPlanner in src/actions/issues.ts (4dd80449) accepts a split only into open issues newer than the hand-over, and accepts done only with a done-on-main note. So finished umbrellas come back to the planner each tick.

Done looks like:

• [ ] handedToPlanner also allows reason: "split" when every issue in into belongs to the same repository, is not the issue itself or the Later gate, and is closed with reason done.
• [ ] Tests in test/crew-parks.test.ts: the planner closes an umbrella as split into done issues. It is refused when one of them is still open, when one was closed as dropped, and when one is in another repository.
• [ ] README's planner paragraph adds this case in one sentence.