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)

docker-compose.yml

@@ -0,0 +1,24 @@
+services:
+  gateway:
+    build: .
+    container_name: gravity-gateway
+    restart: unless-stopped
+    ports:
+      - "8585:8585"
+    environment:
+      - DISCORD_TOKEN=${DISCORD_TOKEN}
+      - DISCORD_GUILD_ID=${DISCORD_GUILD_ID}
+      - BOT_MODE=gateway
+      - GATEWAY_PORT=8585
+      # Brain path inside container (not used in gateway mode, but needed for config validation)
+      - BRAIN_PATH=/app/data/brain
+    volumes:
+      - gateway-data:/app/data
+    logging:
+      driver: json-file
+      options:
+        max-size: "10m"
+        max-file: "3"
+
+volumes:
+  gateway-data: