Expose local files in Codespaces

[krowvin]

Summary

• add the mounted dev-container workspace root to PYTHONPATH
• make checked-out source and local test modules available throughout GitHub Codespaces
• document that the files remain live from the workspace instead of being copied into the image

Why

The post-create Poetry install prepares the project and development dependencies, but processes started outside the repository working directory do not consistently discover repository-local source and test files. Exposing ${containerWorkspaceFolder} through PYTHONPATH makes the local checkout available across the Codespace.

Impact

Edits to source and test files are immediately reflected in Python processes anywhere in the dev container. Normal package installation and the existing virtual environment layout are unchanged.

Validation

• clean dev-container creation, including the existing Poetry post-create install
• import/path check from /tmp confirmed cwmscli and tests resolve under /workspaces/cwms-cli
• poetry run pytest -q (373 passed)
• poetry run pre-commit run --all-files