feat: add reversible workspace panels

docs/UI_SPIKE_SCORECARD.md

@@ -25,8 +25,11 @@ The current programs establish dependency, build, launch, and interaction
 baselines. GPUI now routes a real PTY through a VT engine and keyboard encoder,
 paints coalesced styled cell runs and cursor shapes, and derives PTY rows/columns
 from the 60% terminal region inside each responsive one/three/five-panel layout
-when the window changes. Retained-history page/top/bottom navigation is wired. Text selection,
-mouse modes, a native Markdown editor, and one isolated browser child remain.
+when the window or attached panel count changes. Retained-history
+page/top/bottom navigation is wired. Panel detach/reattach keeps background
+surface updates alive and exposes detached sessions in the sidebar. Text
+selection, mouse modes, a native Markdown editor, and one isolated browser child
+remain.
 
 ## Ubuntu baseline — metal, 2026-08-31