Maity: Board

hub: a merge also closes cards on `blocked`

tsk-aq4fjk5tby1b open fix

Adopted by the maintainer from the jour fixe of 2026-11-02 (round jfx-1fze3cfeq55h, req-k1xwkk57y3fn). Otherwise the card keeps showing open work although its PR is on main. Files: hub/src/maity_hub/merged.py, hub/src/maity_hub/cards.py, docs/api.md (merge closes cards), tests.