docs: add environment variables configuration guide

[melihaltin]

Summary

Added a comprehensive environment variables configuration section to CONTRIBUTING.md to help new contributors set up their development environment more easily. This includes step-by-step instructions for generating security keys, configuring AI model APIs, and setting up OAuth integrations for GitHub and Google services.

Fixes #(issue)

Type of Change

• [ ] Bug fix
• [ ] New feature
• [ ] Breaking change
• [x] Documentation
• [ ] Other: ___________

Testing

✅ Tested on macOS:

• Verified all OpenSSL commands work correctly
• Confirmed OAuth callback URLs format matches the codebase
• Tested that all environment variable names align with existing .env.example
• Validated markdown formatting renders properly

Reviewers should focus on:

• Accuracy of the OAuth setup instructions
• Completeness of required vs optional environment variables
• Clarity of step-by-step instructions for new contributors

Checklist

• [x] Code follows project style guidelines
• [x] Self-reviewed my changes
• [x] Tests added/updated and passing (N/A for documentation)
• [x] No new warnings introduced
• [x] I confirm that I have read and agree to the terms outlined in the Contributor License Agreement (CLA)

Screenshots/Videos

Before: Contributors had no clear guidance on environment setup After: Clear step-by-step environment configuration guide with:

• Security key generation commands
• AI model API key options (OpenAI, Anthropic, Ollama)
• GitHub and Google OAuth setup instructions
• Required vs optional variable distinction

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