refactor(engine): remove follow-up choices from insert-beat, keep multi-beat only

Insert-beat is a pure in-scene micro-interaction — adding choices that
lead to change-scene contradicted its purpose. Now insert-beat generates
1-3 richer beats then loops back to the original options, which is the
natural UX for "you glanced at something decorative."

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/types/index.ts

@@ -695,19 +695,15 @@ export type InsertBeatPartial = {
   lineDelivery?: string;
 };
 
-/** Multi-beat response: 1-3 beats + optional follow-up choices. */
+/** Multi-beat response: 1-3 beats. */
 export type InsertBeatMulti = {
   beats: InsertBeatPartial[];
-  /** Follow-up choices shown after the last beat (max 2). */
-  choices?: { label: string; effect: string }[];
 };
 
 export type InsertBeatResponse = {
   partial: InsertBeatPartial;
   /** Additional beats beyond the first (for richer insert-beat interactions). */
   extraBeats?: InsertBeatPartial[];
-  /** Follow-up choices shown after the last inserted beat. */
-  followUpChoices?: { label: string; effect: string }[];
   characters: Character[];
 };