Clear the search list after a pick, and stop double-announcing tracks

Two bits of clutter visible in one screenshot: the results list stayed in
the channel after a track was chosen, still wearing its reactions, and
every track produced two lines — the command's own reply and the player's
"now playing" notice.

Picking now spends the session and deletes the list, so a second reaction
does nothing and the message goes away. The player announces a track only
when it started one by itself; when a command started it, that command
has already said so.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/bot/commands.ts

@@ -81,13 +81,22 @@ function activeSession(messageId: string | undefined): SearchSession | null {
   return session;
 }
 
-/** Resolves a reaction on a results message into the track it stands for. */
+/**
+ * Resolves a reaction on a results message into the track it stands for, and
+ * spends the session: the list is answered once, so a second reaction on the
+ * same message does nothing and the message itself can go away.
+ */
 export function trackForReaction(messageId: string, userId: string, emoji: string): Track | null {
   const session = activeSession(messageId);
   // Only the person who searched picks; otherwise anyone could hijack the list.
   if (!session || session.userId !== userId) return null;
   const index = CHOICE_EMOJI.indexOf(emoji);
-  return index === -1 ? null : (session.tracks[index] ?? null);
+  const track = index === -1 ? null : (session.tracks[index] ?? null);
+  if (track) {
+    sessionsByMessage.delete(messageId);
+    latestByUser.delete(`${session.channelId}:${session.userId}`);
+  }
+  return track;
 }
 
 function added(track: Track, startedNow: boolean, position: number): string {