fix(collector): 기능 누락 3건 수정 — Discord 명령어/채팅/등록 중계

Gap 1: Discord→Extension 명령어 깨짐
  - bot.py: _write_command() 래퍼 — gateway.push_command()도 호출
  - main.py: bot.gateway 연결
  - 슬래시 명령어 + on_message 모두 _write_command 사용

Gap 2: Chat snapshot 미전달
  - collector.py: _forward_chat_snapshots_loop 추가

Gap 3: Session registration 미전달
  - collector.py: _forward_registrations_loop 추가

main.py

@@ -107,6 +107,7 @@ async def main():
             from gateway import GatewayAPI
             gateway_port = int(os.environ.get('GATEWAY_PORT', '8585'))
             gateway = GatewayAPI(bot, port=gateway_port, api_key=Config.GATEWAY_API_KEY)
+            bot.gateway = gateway  # Enable _write_command → gateway.push_command
             await gateway.start()
             logger.info(f"Gateway API running on port {gateway_port}")