feat(gateway): Docker Gateway 봇 + HTTP API 구현 #task-311

- gateway.py: Collector↔Gateway HTTP API (pending, response, chat, register, commands)
- Dockerfile + docker-compose.yml: BOT_MODE=gateway, port 8585
- main.py: gateway 모드 (watcher 비활성, GatewayAPI 시작)
- config.py: gateway 모드 BRAIN_PATH 검증 스킵
- requirements.txt: aiohttp 추가
- docs/usage-guide.md: Docker 배포 섹션 추가
- Extension VSIX v0.3.9 빌드 (auto-approve 포함)

config.py

@@ -52,6 +52,7 @@ class Config:
             errors.append("DISCORD_TOKEN is not set")
         if not cls.DISCORD_GUILD_ID:
             errors.append("DISCORD_GUILD_ID is not set")
-        if not cls.BRAIN_PATH.exists():
+        # Gateway mode doesn't need local BRAIN_PATH
+        if cls.BOT_MODE != 'gateway' and not cls.BRAIN_PATH.exists():
             errors.append(f"BRAIN_PATH does not exist: {cls.BRAIN_PATH}")
         return errors