Add GitHub Actions CI

[Copilot]

Adds a simple CI pipeline via GitHub Actions that runs on every push and pull request.

Workflow steps

• Lint with flake8
• Test with pytest

Notes

• Python 3.13
• Poetry installed via pipx install poetry with dependency cache enabled via setup-python cache: 'poetry'
• GITHUB_TOKEN scoped to contents: read

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[SupraSummus]

@copilot use py 3.13 and modern setup python. see https://github.com/actions/setup-python/blob/main/docs/advanced-usage.md#caching-packages

[Copilot]

@copilot use py 3.13 and modern setup python. see https://github.com/actions/setup-python/blob/main/docs/advanced-usage.md#caching-packages

Done in a60e923. Switched to Python 3.13 and the modern caching approach from the docs: Poetry is now installed first via pipx install poetry, then setup-python runs with cache: 'poetry' — removing the need for snok/install-poetry.