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)

.dockerignore

@@ -0,0 +1,9 @@
+__pycache__/
+*.pyc
+.env
+*.vsix
+extension/node_modules/
+extension/out/
+.deps_installed
+gravity_control.log
+*.tar.gz