docs: add CI workflow, CONTRIBUTING.md, and project structure guide

[dolepee]

Summary

Adds foundational CI infrastructure and contributor documentation to make the project more accessible and maintainable.

Changes

1. GitHub Actions CI workflow (.github/workflows/ci.yml)

• Runs on push/PR to main
• Lint job: Catches syntax errors and undefined names via flake8 (hard fail), plus style warnings (non-blocking)
• Import check job: Validates all Python files have correct syntax using ast.parse

2. CONTRIBUTING.md

• Prerequisites and setup instructions
• Project structure overview explaining what each module does
• Contribution guidelines (issues, PRs, code style)
• How to run experiments with parameter reference table

Motivation

The project currently has no CI or contributor docs. As an ICML 2025 paper implementation, it's likely to attract community interest. These additions lower the barrier to entry for new contributors and catch basic errors automatically.

[dolepee]

Hi team, could you please share feedback on this PR when you have a chance? I can apply changes quickly. Thank you.