docs: add comprehensive GitHub repository setup guide

Added GITHUB_SETUP.md (620 lines) covering:
- Required secrets (ANTHROPIC_API_KEY, PROJECTS_TOKEN)
- Branch protection rules (main and dev) with CLI commands
- GitHub Projects v2 setup with exact status field values
- Repository settings and GitHub Actions permissions
- Labels configuration with colors and descriptions
- Optional settings (GitHub Pages, Dependabot, Environments)
- Complete validation checklist with test commands
- Troubleshooting guide for common issues
- Automated setup script template

Updated README.md to link to GITHUB_SETUP.md in Quick Start section.

This provides complete documentation for all GitHub repository
configuration requirements, answering the user's question about
required settings for branch protection, project board, and other
GitHub configuration.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: alirezarezvani