ci: add automated Ruff linting and formatting

[Huoyanlifusu]

Summary

Related to #124.

This draft adds the initial Ruff configuration, CI checks, and local usage instructions.

While validating the change, I found that applying Ruff repository-wide exposes a large amount of existing style debt. It also reports expected errors in learner exercises, shared-namespace checks, and intentionally invalid test fixtures. I have intentionally not reformatted the repository or added extensive repository-specific exclusions yet, to keep this PR focused.

Tests

zhangyuyang@zhangyuyangdeMacBook-Pro pythonlings % python -m pytest -q
.......................................................................................................................................................................... [ 80%]
..........................................                                                                                                                                 [100%]
212 passed in 40.27s

Screenshots

Checklist

• Updated docs when behavior changed
• Added or updated tests
• Verified python -m pytest -q

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[Huoyanlifusu]

@abhiksark Before making the CI check required, would you confirm whether you prefer:

1. establishing a repository-wide Ruff formatting baseline in this PR; excluding curriculum and fixture paths; or
2. introducing Ruff incrementally and handling the existing violations in a follow-up issue?

[abhiksark]

2nd makes more sense.

[abhiksark]

Thanks for putting together the initial Ruff configuration and contributor documentation. The focused scope, Python 3.9 target, 80-character limit, and development-only dependency all look good.

There is one blocker before this can merge: the lint job sets up Python but never installs Ruff. The current Actions log shows both commands failing with ruff: command not found; because each step has continue-on-error: true, the job still appears green even though Ruff never runs.

Please add a non-advisory installation step before the checks, for example:

- name: Install development dependencies
  run: python -m pip install -e ".[dev]"

The two Ruff commands can remain advisory for this incremental phase. After making that change, please update the branch from the current dev and rerun CI, since the branch is currently behind the latest multi-platform workflow.

One non-blocking note: the current configuration reports substantial existing debt, including expected shared-namespace errors in checks/. That is reasonable for temporary advisory scaffolding, but it does not yet satisfy #124's final requirement that lint failures block CI. Keeping this PR as “related to #124” and handling the baseline/per-file ignores in follow-up work makes sense.

[Huoyanlifusu]

@abhiksark Thanks! I’ve added the development dependency installation step.