An agent that runs all its sub-agents concurrently.
Each sub-agent runs in its own Task with an isolated branch. Events from
all sub-agents are collected and returned in a single flat list.
Branch naming follows the pattern "parent.sub" (or "existing.parent.sub"
if the context already has a branch set).
Since sub-agents run concurrently, the order of events across sub-agents is nondeterministic.