style(sidebar): add smooth transition to New Agent button hover

150ms transition for border-color and background-color.

🤖 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

@@ -354,6 +354,8 @@ const styles = StyleSheet.create((theme) => ({
     borderRadius: theme.borderRadius.lg,
     borderWidth: 1,
     borderColor: theme.colors.border,
+    transitionProperty: "border-color, background-color",
+    transitionDuration: "150ms",
   },
   newAgentButtonHovered: {
     borderColor: theme.colors.foreground,