docs: add persistent storage documentation and dynamic versioning

[DEENUU1]

[0.2.12] - 2025-01-16

Changed

• Updated pydantic-ai-backend dependency to >=0.0.4 for persistent storage support
• __version__ now dynamically reads from package metadata (pyproject.toml) via importlib.metadata

Documentation

• Added persistent storage documentation to docs/examples/docker-sandbox.md:
  • volumes parameter for DockerSandbox
  • workspace_root parameter for SessionManager
• Added workspace_root documentation to docs/examples/docker-runtimes.md:
  • Configuration options section
  • New "Persistent Storage with workspace_root" section with examples
  • Directory structure diagram
• Updated docs/examples/full-app.md with workspace_root in SessionManager example
• Updated examples/full_app/app.py to use workspace_root for persistent user files

#18