swarm-ui: shared form-field kit (TextField, SelectField, Button)

Closes #3448.

New ui/ primitives: FormField (shared label+control wrapper),
TextField, SelectField, Button — each with a min-height touch target
(2.75em ~= 44px, WCAG 2.5.5) per mara's #3447 ask, and a max-width
instead of a fixed width so the control caps on desktop without
overflowing a narrow/touch viewport.

CreateAgentPage's name field + submit button now come from the kit
instead of page-scoped CSS; ComponentsPage gets a section for each new
primitive with an editable sample.

frontend/packages/swarm-ui/src/ui/select-field/SelectField.tsx

@@ -0,0 +1,46 @@
+// <SelectField> — a labelled `<select>`, `TextField`'s sibling for the
+// "pick one of these" shape (the hive-choice dropdown the create-agent
+// form needs is the motivating caller). Options are plain
+// value/label pairs, not `ComponentChildren` — every real caller so far
+// has flat string options, and generic children would need a second
+// primitive (`SelectField.Option`) for zero real benefit today.
+import { FormField } from '../form-field/FormField.js';
+
+export interface SelectOption {
+  value: string;
+  label: string;
+}
+
+export function SelectField({
+  id,
+  label,
+  value,
+  onChange,
+  options,
+  required,
+}: {
+  id: string;
+  label: string;
+  value: string;
+  onChange: (value: string) => void;
+  options: SelectOption[];
+  required?: boolean;
+}) {
+  return (
+    <FormField label={label} htmlFor={id}>
+      <select
+        id={id}
+        class="ui-form-control"
+        value={value}
+        required={required}
+        onChange={(e) => onChange((e.target as HTMLSelectElement).value)}
+      >
+        {options.map((o) => (
+          <option key={o.value} value={o.value}>
+            {o.label}
+          </option>
+        ))}
+      </select>
+    </FormField>
+  );
+}