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/text-field/TextField.tsx

@@ -0,0 +1,44 @@
+// <TextField> — a labelled single-line text input, the shared control
+// every page-level form (`CreateAgentPage` today) reaches for instead
+// of hand-rolling its own label/input pair. No textarea/multi-line
+// mode — promote that the day a real caller needs one, same "don't
+// build ahead of a caller" rule the rest of `ui/` follows.
+import { FormField } from '../form-field/FormField.js';
+
+export function TextField({
+  id,
+  label,
+  value,
+  onInput,
+  type = 'text',
+  pattern,
+  title,
+  required,
+  placeholder,
+}: {
+  id: string;
+  label: string;
+  value: string;
+  onInput: (value: string) => void;
+  type?: string;
+  pattern?: string;
+  title?: string;
+  required?: boolean;
+  placeholder?: string;
+}) {
+  return (
+    <FormField label={label} htmlFor={id}>
+      <input
+        id={id}
+        class="ui-form-control"
+        type={type}
+        value={value}
+        pattern={pattern}
+        title={title}
+        required={required}
+        placeholder={placeholder}
+        onInput={(e) => onInput((e.target as HTMLInputElement).value)}
+      />
+    </FormField>
+  );
+}