Adds:
- TaskService.reassign unit tests (set, clear, missing-task)
- New test_choreographer_reassignment.py covering:
i_am_done -> qa
pass_review -> documenter
i_documented -> cell_pm
main_pm_complete -> None (CEO via UI)
escalate_to_ceo (board) -> None
cell_pm_complete -> walks up to parent and reassigns when all
siblings terminal, skips otherwise
fail_review -> does NOT issue an explicit reassign (qa_fail
already restores the original developer via
quick_context)
