swarm-ui: groups of agent terminals: drag a terminal tab onto a terminal to split it, tmux style, with resizable panes, a split button, and a name you can edit

docs/swarm/ui.md

@@ -157,6 +157,28 @@ popovers on table headers work the same way. You can stop or pause the
 agent, or link an account, without leaving the terminal. A subagent's tab opens
 its agent's details.
 
+#### Groups of terminals
+
+Terminals can sit side by side in a group, split the way tmux splits panes. Drag
+another agent's terminal tab from the header onto the left, right, top or
+bottom half of a terminal: the half it would take lights up, and the drop makes
+a group of the two, with its own header tab at `/agents/group/<id>`. In a group,
+the same drag onto a pane splits that pane, as many times as you like, and
+dragging a pane's header badge onto another pane moves it there. The **split**
+button in a pane's header does the same for an agent whose terminal tab is open:
+**right** or **below** puts it beside the pane. The **×** takes the pane's
+agent out of the group, and a group left with one agent goes, landing on that
+agent's own terminal. Drag a divider to resize, or focus it and use the arrow
+keys. Panes have no minimum size; a divider stops at 2% so a pane can't
+vanish.
+
+A group's tab shows its agents, `foo + bar`. Double-click it to give it a
+name; clear the name to go back to the agents. Closing the tab closes the
+group. An agent in a group still has its own terminal tab, and you can open it
+full screen a second time. Groups and their layouts live in this browser
+only, like the open terminal tabs, so they survive a reload but aren't shared
+with another browser.
+
 ### Review
 
 The **review** card on the **jobs** tab is the one place for the