feat: add Mixture of Agents (MoA) for arbiter verdicts

When MOA_ENABLED=true, arbiter queries multiple LLM models in parallel
via OpenAI-compatible API, then an aggregator model synthesizes the
final verdict from all opinions. Falls back to single-agent arbiter
when MoA is disabled.

Config: MOA_BASE_URL, MOA_API_KEY, MOA_REFERENCE_MODELS, MOA_AGGREGATOR_MODEL

src/message-agent-executor.test.ts

@@ -25,6 +25,20 @@ vi.mock('./config.js', () => ({
     effort: undefined,
     fallbackEnabled: true,
   })),
+  getMoaConfig: vi.fn(() => ({ enabled: false, referenceModels: [], aggregator: {} })),
+  TIMEZONE: 'Asia/Seoul',
+}));
+
+vi.mock('./arbiter-context.js', () => ({
+  buildArbiterContextPrompt: vi.fn(() => ''),
+}));
+
+vi.mock('./moa.js', () => ({
+  runMoaArbiter: vi.fn(),
+}));
+
+vi.mock('./platform-prompts.js', () => ({
+  readArbiterPrompt: vi.fn(() => ''),
 }));
 
 vi.mock('./db.js', () => ({