feat: add responsive ultrawide work lanes

docs/PRODUCT_SPEC.md

@@ -66,11 +66,13 @@ surface change never silently launches a process, moves the pane to another
 host, or changes which agent owns the session. Unsupported surfaces are shown as
 unavailable rather than simulated.
 
-The default workspace is a 3×1 vertical composition: the top 20% shows context,
-pane/agent state, tools, and goal; the middle 60% is the selected terminal or
-work surface; the bottom 20% is the decision shelf. Multiple panes may continue
-working offscreen and remain one shortcut away. Explicit split and dashboard
-layouts remain supported, but they do not displace the primary-terminal default.
+The default workspace has three horizontal bands: the top 20% shows context,
+pane/agent state, tools, and goal; the middle 60% is a responsive set of vertical
+work lanes; and the bottom 20% is the decision shelf. The work band shows one
+lane in compact windows, three at normal desktop widths, and up to five on an
+ultrawide display. One lane owns keyboard input at a time, and selecting a lane
+never pauses the others. Explicit split trees and dashboard layouts remain
+supported, but they do not displace this terminal-first default.
 
 An optional decision shelf sits below the active work surface. It may show an
 answer draft, two or three concrete choices, why each was suggested, and the