Create diagram and writeup for tasks and task-coordinator (agent replacement)

[pjnorton]

Agent-Subagent needs redesign to be more capable (and potentially developer friendly).

Design doc here: https://docs.google.com/document/d/19hYu7ZbUQWMTCqviu2BIKhDN7mweiSEjC0alDRScRsM/edit