docs(native-chat): record why the question answer row has no slash grammar (#21084)

The free-text row on an AskUserQuestion card is a plain input on purpose,
but nothing said so, and its absence reads as a missing picker rather than
a decision. Note the reason at the input.

src/renderer/src/components/native-chat/NativeChatQuestionCard.tsx

@@ -192,6 +192,10 @@ export function NativeChatQuestionCard({
                   <span className="flex size-6 shrink-0 items-center justify-center rounded-md bg-muted text-muted-foreground">
                     <Pencil className="size-3.5" />
                   </span>
+                  {/* Intentionally plain — no `/` or `@` grammar. This row answers the
+                      question; a slash command addresses the session, so running one here
+                      could only answer with command text or abandon the pending prompt.
+                      That grammar belongs to the composer, which this card replaces. */}
                   <input
                     ref={answerInputRef}
                     disabled={isSubmitting}