feat(ui): search in the agent model dropdown (#352)

Select gains an optional search box that filters its options; the agent model pickers use it, so long model lists can be filtered instead of scrolled. Keyboard navigation and type-ahead work on the filtered list, and the popup keeps its width while filtering.

Co-authored-by: dimassfeb-09 <[email protected]>

src/features/agents/ConfigPickers.tsx

@@ -44,6 +44,7 @@ export function ConfigPickers({
                     : undefined,
               detail: v.description ?? undefined,
             }))}
+            searchable={o.category === "model"}
             onChange={(v) => onChange(o.id, v)}
             // Effort and other levels say what they are: "Effort  Medium".
             renderValue={