ci: add automated style linting (ruff) so style issues are caught before review

[Huoyanlifusu]

Problem

Style rules are currently enforced by hand during review. For example, #121 was asked to re-wrap lines to 80 chars and add the file-location comment (# checks/async/async7.py) after the fact — things a linter could catch before a maintainer spends time on the PR.

Current state

• No ruff/black/flake8/isort config, no .editorconfig, no .pre-commit-config.yaml.
• pyproject.toml has no [tool.ruff] section.
• CI (ci.yml) runs pytest, the passing-curriculum verify, and python -m build — but no lint step.

Proposal

Add ruff as the single linter and formatter, configured for the repo's existing Google-Python-style / 80-character convention:

• [tool.ruff] with line-length = 80
• ruff check and ruff format --check as a CI job
• ruff added to the dev dependency group
• optionally a .pre-commit-config.yaml for local hooks

Questions for maintainer

• OK to standardize on ruff (instead of flake8 + black)?
• Should the # checks/<topic>/<file>.py file-location comment also get an automated check, or just be documented in CONTRIBUTING.md?

Acceptance criteria

• Lint/format runs in CI and fails the build on style violations.
• Config matches the documented 80-char / Google Python style.
• CONTRIBUTING.md explains how to run lint locally.

[abhiksark]

Hi @Huoyanlifusu
Will you take this?

I can take this as well.

[Huoyanlifusu]

Hey @abhiksark , I'd like to adpot this task.