feat(agents): show the model list and effort in one popover (#2213)

Choosing an agent model took two popovers, and the list cut a name on one row while showing it in full on another. The chip's popover now shows the effort slider above a searchable model list. Names with a description keep their full width, and the description is cut instead. The list is extracted from `Select` as `SelectListbox`; `Select` keeps its previews across popup open and close.

Co-authored-by: panda-student <[email protected]>

src/ui/Select.module.css

@@ -99,6 +99,7 @@
 }
 /* The label keeps its width (up to most of the row); a long detail (a path) is cut instead. */
 .withDetail {
+  flex-shrink: 0;
   max-width: 70%;
 }
 .detail {