The menu on an agent's terminal page used to open the subagent on a tab of
its own. It now starts a group of this terminal and the subagent's, split to
the right; in a group's pane it splits that pane. A pane's layout key is the
agent, or `agent/subagent` for a subagent's pane, so the layout operations
stay as they are and a destroyed agent takes its subagent panes with it. A
subagent already in the group shows a marker and does nothing when chosen,
and maximize opens a subagent pane's terminal on its own.
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