feat(ui): let users hide context rail surfaces

A trailing configure button on the rail — outside the sortable list and the
digit shortcuts — opens a dialog that toggles each surface. The choice is
stored as the hidden set so newly added surfaces appear for everyone, and
the rail and the mod+alt+digit switcher share the same visibility filter, so
badges and shortcuts always agree. Hidden surfaces keep their data and stay
reachable from the command palette.

packages/ui/src/lib/surfaces/DOCUMENTATION.md

@@ -22,7 +22,10 @@ edge (`components/layout/ContextPanelRail.tsx`) and rendered by
   registry's default order and appends any missing surfaces.
 - `getVisibleContextRailSurfaces` is the single visibility filter shared by the
   rail and the global surface-switch shortcut (`switch_context_surface` in
-  `lib/shortcuts.ts`): it drops the plan surface unless plan mode is enabled,
+  `lib/shortcuts`): it drops surfaces the user hid
+  (`useUIStore.contextRailHiddenSurfaces`, edited from the rail's trailing
+  configure button — `ContextRailSurfacesDialog`), drops the plan surface
+  unless plan mode is enabled,
   drops the walkthrough on VS Code and below `WALKTHROUGH_MIN_WIDTH`, and hides
   `has-content` surfaces until a tab of their mode exists. Both consumers use
   it so the digit shown on a rail badge always maps to the same surface the

packages/ui/src/lib/surfaces/registry.ts

@@ -187,6 +187,9 @@ export const sortContextSurfaces = (railOrder: readonly string[]): ContextSurfac
 
 type VisibleRailSurfacesOptions = {
   railOrder: readonly string[];
+  /** Surfaces the user chose to hide from the rail (and from the digit
+      shortcuts, which share this filter). */
+  hiddenSurfaces?: readonly string[];
   planModeEnabled: boolean;
   isVSCode: boolean;
   screenWidth: number;
@@ -203,6 +206,9 @@ type VisibleRailSurfacesOptions = {
  */
 export const getVisibleContextRailSurfaces = (options: VisibleRailSurfacesOptions): ContextSurfaceDescriptor[] => {
   return sortContextSurfaces(options.railOrder).filter((surface) => {
+    if (options.hiddenSurfaces?.includes(surface.id)) {
+      return false;
+    }
     if (surface.id === 'plan' && !options.planModeEnabled) {
       return false;
     }