Define CrewAI Agent Roles and Goals

Using CrewAI, define three agents: an 'AI Researcher', a 'Robotics Engineer', and a 'Project Manager'. Assign them distinct backstories, roles, and overarching goals relevant to designing an autonomous excavation AI. Specify the tools each agent should have access to (e.g., 'Web Search Tool', 'Milvus DB Tool', 'Document Writer Tool'). Provide Python code for the agent definitions.