Lands the rest of the dialog redesign stack, all behind the new-designs experiment:
- put dialog search in the title row (#55800)
- hide dialog search on short lists (#55801)
- read the experiment through useNewDesigns (#55802)
- raise dialog footers (#55805)
- standardize dialog footer actions (#55806)
- style selected dialog list rows (#55807)
- show list overflow indicator (#55808)
- restyle dialog headers (#55809)
- use vivid agent colors (#55810)