docs: Add CONTRIBUTING.md

[xingzihai]

Summary

This PR adds a comprehensive CONTRIBUTING.md file to help contributors understand how to contribute to the last30days-skill project.

What's Included

Contribution Guidelines

• Code of Conduct - Contributor Covenant reference
• Bug reporting template - Clear format for reporting issues
• Feature request template - Structured format for suggestions
• Pull Request workflow - Step-by-step guide for submitting PRs

Development Setup

• Prerequisites (Python 3.10+, Node.js 22+)
• Clone and setup instructions
• API key configuration
• Test verification steps

Code Style Guidelines

• Type hints for all function signatures
• Dataclasses for structured data
• Error handling best practices
• Logging with the logging module
• Documentation with docstrings
• No external dependencies - pure Python stdlib

Testing

• Running tests with pytest
• Writing test patterns and examples
• Test categories (unit, integration, smoke, mock)

Project Documentation

• Complete project structure breakdown
• Guide for adding new data sources
• Security considerations for API key handling

Why This Matters

The project had no CONTRIBUTING.md, making it harder for new contributors to understand:

• How to set up the development environment
• What code style to follow
• How to run and write tests
• The proper workflow for submitting contributions

This guide addresses all of these gaps with clear, actionable instructions tailored to the project's specific technology stack (Python stdlib-only, multi-source research tool).

Testing

• File follows standard CONTRIBUTING.md format
• All code examples are valid Python
• Project structure matches actual repository
• Links to GitHub issues/PRs are correct

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Thank you for considering this contribution! 🎉

[mvanhorn]

See my reply on #109 covering all your PRs. Can't commit to anything right now with the v3.0 refactor underway, but I promise I'll consider each one once it lands. Appreciate the work.