feat(layout): add tabbed window groups and scrolling columns

docs/shortcuts.md

@@ -49,14 +49,23 @@ two most recently focused windows.
 | --- | --- |
 | Super + R | Cycle column width |
 | Super + C | Center the focused column |
-| Super + [ | Place the window in a neighbouring column |
-| Super + ] | Extract the window into its own column |
+| Super + [ | Join a neighbouring column or tree tab group |
+| Super + ] | Leave the column or tree tab group |
+| Super + Ctrl + [ / ] | Previous / next tab in a group |
+| Super + Shift + T | Toggle a scrolling column between stacked rows and tabs |
 | Super + M | Switch scrolling / tree layout |
 | Super + Tab | Open or close overview |
 
 In scrolling mode, **Super + [** uses the left neighbour when available, otherwise
 the right. Try it with two terminals to place them vertically in one column.
-**Super + ]** separates the focused window again.
+**Super + ]** separates the focused window again. **Super + Shift + T** changes
+the column to tabs or restores its stacked rows. In tabbed columns, **Super + J/K**
+switches tabs; the usual left/right focus keys still switch columns.
+
+In tree mode, **Super + [** joins the focused window to a neighbouring window
+as a tab. Click a title in the tab bar or use **Super + Ctrl + [ / ]** to switch
+tabs. **Super + ]** splits the focused tab out beside the group. For a specific
+neighbour, bind `join-tab` with a direction; see [Commands and bindings](configuration.md#commands-and-bindings).
 
 In overview, select a workspace to see its windows, then click the window you
 want. Directional focus keys move selection; Enter activates it and Escape closes