feat(agents): group model picker by provider and improve search (#486)

Agents can report model ids with nested provider paths, which made long rows that lost their provider context. The model picker now shows the short name under a provider heading with the remaining path as detail. Searchable Select matches several tokens across label, detail and hidden keywords, with fuzzy matching, and keeps a heading while one of its items matches.

Select now has an explicit section option, separate from disabled, so a heading can never be selected and a disabled option is not rendered as a heading. Existing section rows in provider options and Find in Files scopes use it. Keyboard navigation skips headings in filtered lists.

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

src/ui/Select.module.css

@@ -136,3 +136,7 @@
   box-shadow: inset 0 0 0 1px var(--border-button);
   color: var(--fg-strong);
 }
+
+.option {
+  display: contents;
+}