feat: add 灵犀 Feishu bot + fix agent schedule system + default all tools

- Add 灵犀学习助手 Feishu bot (lingxi_app_service + lingxi_ws_handler)
- Fix agent_schedule_service missing AgentSchedule import (Celery Beat)
- Fix scene_templates default enable_tools=False → True
- Fix workflow_engine LLM node: empty tools list now = all tools (consistent with agent node)
- Add 创建agent.md guide document

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/services/agent_schedule_service.py

@@ -95,6 +95,8 @@ def check_and_run_due_schedules() -> int:
     Returns:
         本次触发的任务数
     """
+    from app.models.agent_schedule import AgentSchedule
+
     db: Optional[Session] = None
     try:
         db = SessionLocal()