chore: batch Dependabot updates

[yousefkotp]

Summary

• combine Dependabot PRs chore(deps): bump actions/checkout from 4 to 6 #2, chore(deps): bump actions/setup-python from 5 to 6 #3, chore(deps): update huggingface-hub requirement from <1,>=0.20 to >=0.20,<2 #4, and chore(deps-dev): bump ruff from 0.15.7 to 0.15.8 #5 into one update
• bump actions/checkout to v6 and actions/setup-python to v6 in CI
• widen huggingface-hub to <2, bump ruff to 0.15.8, and point CITATION.cff url at the project webpage

Validation

• local isolated Conda env on Python 3.12
• make check
• python -c "from moozy.hf_hub import ensure_tasks_dir; print(ensure_tasks_dir())"