docs: Add Pydantic AI sandbox agent example

[hsteude]

Summary

• Add a lightweight Pydantic AI example that exposes prokube Agent Sandboxes as agent tools
• Include tools for Python execution, shell commands, and file read/write/list operations under /workspace
• Document managed Lab and external API-key configuration

Testing

• python -m py_compile sandboxes/pydantic-ai-code-agent/agent.py
• git diff --check
• PYTHONPATH=sandboxes/pydantic-ai-code-agent OPENAI_API_KEY=test-key uv run --with 'pydantic-ai>=2.0.0' --with 'git+https://github.com/prokube/prokube-sdk.git@v0.1.3' python -c 'import agent; a=agent.build_agent(); print(type(a).name)'

Note: I did not run a live LLM completion because that requires model credentials.