chore: add CONTRIBUTING, CODE_OF_CONDUCT, and CI workflow

[dafahaha]

Summary

This PR adds standard community files to help new contributors understand how to participate and ensure code quality through automated testing.

Changes

1. CONTRIBUTING.md: Contribution guidelines, code style (PEP 8), testing requirements, PR process, and issue reporting template
2. CODE_OF_CONDUCT.md: Contributor Covenant v2.0, providing a clear code of conduct for the community
3. .github/workflows/ci.yml: GitHub Actions CI workflow with:
   • Python 3.10, 3.11, 3.12 matrix testing
   • pip dependency caching
   • flake8 linting (E9, F63, F7, F82 critical errors)
   • pytest test runner (with graceful fallback if no tests exist yet)

Motivation

These standard files are essential for:

• Helping new contributors get started quickly
• Ensuring code quality through automated CI
• Establishing community norms and expectations
• Making the project more welcoming and professional

Testing

• All files are standard templates commonly used in open-source projects
• CI workflow is configured to run on push and PR to main branch
• No functional code changes, only documentation and CI configuration