Add GitHub Actions workflow to check for trailing spaces

[dandye]

Summary

• Adds a new GitHub Actions workflow that automatically checks for trailing spaces in code files during pull requests
• Updates the CONTRIBUTING guide with information about code quality checks
• Fixes issue Add GitHub status check to prevent trailing spaces in code #104

Changes Made

• New workflow: .github/workflows/check-trailing-spaces.yml

  • Runs on all pull requests (opened, synchronized, reopened)
  • Checks Python, Markdown, JSON, YAML, and other text files
  • Provides clear error messages showing which files have trailing spaces
  • Includes helpful commands to fix trailing spaces

• Documentation update: Updated CONTRIBUTING file

  • Added section about automated code quality checks
  • Provides instructions for removing trailing spaces

Test Plan

• Validated YAML syntax of the workflow file
• Tested file detection logic
• Verified workflow triggers are appropriate for PR checks
• Will be automatically tested when this PR is created

Benefits

• Prevents trailing spaces from being committed, reducing unnecessary Git diffs
• Maintains consistent code formatting across the repository
• Provides clear feedback to contributors about formatting issues
• Automated enforcement reduces manual review burden

Fixes #104