Split off the settled part of an idea as a task

When part of an idea is settled and part is still open, the idea's agent can now suggest splitting off the settled part. A new button, "Split off a task", then turns that part into a running task, and the idea stays open with its planned prototypes for the rest. Before, the agent could recommend building while prototypes were still pending. Building then ended the whole idea and silently dropped the prototypes, and the only way to keep them was to plan a full project. Now the owner can start building right away and keep exploring what's still open.