refactor: extract pick() helper, deduplicate random selection

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

server/src/engine/fight-loop.ts

@@ -1,6 +1,7 @@
 import { db, schema } from '../db/index.js'
 import { runMockFight } from './mock.js'
 import { eq } from 'drizzle-orm'
+import { pick } from '../lib/utils.js'
 import { fightEvents } from './events.js'
 
 export interface FightResult {
@@ -157,8 +158,7 @@ function pickMatchup(
   const sorted = [...bots].sort((a, b) => b.eloRating - a.eloRating)
 
   if (style === 'elo_close' || (style === 'mixed' && fightNum % 3 !== 0)) {
-    const idx = Math.floor(Math.random() * bots.length)
-    const bot = bots[idx]
+    const bot = pick(bots)
     const others = bots.filter(b => b.id !== bot.id)
     others.sort((a, b) => {
       const diffA = Math.abs(a.eloRating - bot.eloRating) + Math.random() * 150