Splits chat area with a 30-char right panel listing sub-agent runs.
Yellow border highlights the focused pane. Tab cycles focus; j/k
navigates agents when panel focused; Enter drills into sub-agent
transcript. Shows "No sub-agents" when list is empty.
Closes claudbg-9c8r
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>