fix: decouple timeline from workspace layout

The regression recurred because a floating timeline height was still exported as shared layout space and its z-index exceeded artifacts and notices. Reserve zero layout height and define the timeline below artifact and notification overlays.

src/features/workbench/map-workbench-page.tsx

@@ -925,13 +925,7 @@ export function MapWorkbenchPage({
         {
           ...WORKBENCH_LAYOUT_CSS_VARIABLES,
           "--workbench-agent-current-width": `${currentAgentWidth}px`,
-          "--workbench-timeline-height": `${
-            !timelineVisible
-              ? 0
-              : !isLargeScreen
-                ? OPERATIONAL_TIMELINE_LAYOUT.mobileSummaryHeight + timelineBottom
-                : OPERATIONAL_TIMELINE_LAYOUT[timelineMode].height + timelineBottom
-          }px`
+          "--workbench-timeline-height": `${OPERATIONAL_TIMELINE_LAYOUT.overlayReservedHeight}px`
         } as CSSProperties
       }
     >
@@ -1094,7 +1088,7 @@ export function MapWorkbenchPage({
         {shouldShowTaskTicker ? (
           <motion.div
             key="agent-task-ticker-shell"
-            className="fixed left-1/2 right-auto top-24 z-20 w-[calc(100%-1.5rem)] [translate:-50%_0] md:absolute md:w-[var(--workbench-ticker-width)] 2xl:w-[var(--workbench-ticker-width-wide)]"
+            className="fixed left-1/2 right-auto top-24 z-50 w-[calc(100%-1.5rem)] [translate:-50%_0] md:absolute md:w-[var(--workbench-ticker-width)] 2xl:w-[var(--workbench-ticker-width-wide)]"
             style={{ transformOrigin: "top center" }}
             initial={
               prefersReducedMotion