Document Hugging Face dataset download helper

[anth-volk]

Fixes #351

Summary

• Document the Hugging Face repository identifier, repository-relative file path, revision, and local destination accepted by download_huggingface_dataset.
• Document that the function returns the downloaded file's local path.
• Correct the helper's nullable revision, destination path, repository-privacy, and authentication-token annotations without changing runtime behavior.
• Add the required changelog fragment.

Verification

• UV_PYTHON=3.13 uv run ruff format --check policyengine_core/tools/hugging_face.py
• UV_PYTHON=3.13 uv run ruff check policyengine_core/tools/hugging_face.py
• UV_PYTHON=3.13 uv run mypy --config-file mypy.ini policyengine_core/tools/hugging_face.py
• UV_PYTHON=3.13 uv run pytest tests/core/tools/test_hugging_face.py -q (15 passed)

Documentation review

• Documentation change: the public helper's source documentation now specifies its complete input and output contract; no separate guide or generated artifact changes are required.
• Impact: low. This changes annotations and documentation only.
• Confidence: high, based on focused tests, formatting, linting, and static typing.
• Known gaps: none.