refactor(agent-office): derive ACTIVE_AGENT_IDS from GRID_SLOTS

Eliminates dual-write drift risk between ACTIVE_AGENT_IDS list
and GRID_SLOTS slot ordering. Single source of truth is now
GRID_SLOTS; ACTIVE_AGENT_IDS is computed from it.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/pages/agent-office/constants.js

@@ -14,8 +14,6 @@ export const AGENT_META = {
   lotto:      { displayName: '로또 큐레이터',   color: '#ef4444', image: lottoImg },
 };
 
-export const ACTIVE_AGENT_IDS = ['stock', 'music', 'insta', 'realestate', 'lotto'];
-
 // 3x3 슬롯 (좌→우, 위→아래). 처음 5칸은 active, 나머지 4칸은 placeholder.
 export const GRID_SLOTS = [
   { agentId: 'stock' },
@@ -29,6 +27,8 @@ export const GRID_SLOTS = [
   { agentId: null },
 ];
 
+export const ACTIVE_AGENT_IDS = GRID_SLOTS.filter(s => s.agentId !== null).map(s => s.agentId);
+
 export const PLACEHOLDER_IMAGE = undeterminedImg;
 export const PLACEHOLDER_LABEL = '준비 중';