style(sidebar): use darker border color on New Agent hover

Changed from white to zinc[500] for a subtler effect.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

packages/app/src/components/sliding-sidebar.tsx

@@ -358,7 +358,7 @@ const styles = StyleSheet.create((theme) => ({
     transitionDuration: "150ms",
   },
   newAgentButtonHovered: {
-    borderColor: theme.colors.foreground,
+    borderColor: theme.colors.palette.zinc[500],
     backgroundColor: theme.colors.muted,
   },
   newAgentButtonText: {