Add server self-description tool and Chinese FAQ entry

[Codex]

Users asked “你是谁，有什么功能” and the server needed a concise self-description endpoint plus documentation in Chinese.

• Server overview API: Added LAMMPSServer.get_overview() with English/Chinese summary, component status, and capability categories; registered new describe_server MCP tool via info_tools.
• Docs update: Added FAQ entry in README.md answering the identity/capability question in Chinese.
• Tests & resilience: Extended server tests to cover the overview and use tmp work dirs; added OpenFF optional-import fallbacks.

Example:

from mcp_lammps.server import LAMMPSServer
server = LAMMPSServer()
overview = server.get_overview()
# overview["summary_zh"]: "我是一个用于控制 LAMMPS 分子动力学模拟的 MCP 服务器..."