Support --pull-changes-to-fork option for upstream sync

[konard]

🎯 Summary

This PR implements the --pull-changes-to-fork option requested in issue #16, allowing users to synchronize their forks with the original (upstream) repositories.

📋 Issue Reference

Fixes #16

✨ What's New

New CLI Option: --pull-changes-to-fork

• Detects which repositories are forks using GitHub API data
• Sets up or updates upstream remote to point to the parent repository
• Fetches latest changes from the upstream repository
• Merges upstream changes into the local fork
• Pushes synchronized changes back to the fork

🔧 Implementation Details

Core Changes

• Enhanced GitHub API integration: Extended getOrganizationRepos() and getUserRepos() functions to include fork metadata (fork, parent properties)
• New sync functions: Added syncForkWithUpstream() and syncWithUpstreamBranch() for upstream synchronization
• Safe operation: Checks for uncommitted changes before sync, preventing data loss
• Error handling: Graceful handling of merge conflicts with informative messages
• Protocol support: Works with both SSH and HTTPS clone URLs

CLI Integration

• Added --pull-changes-to-fork boolean option with proper validation
• Updated help text and usage examples
• Integrated into existing repository processing pipeline
• Works with existing threading and live updates systems

Example Usage

# Sync all forks in a user account with their upstream repositories
gh-pull-all --user myusername --pull-changes-to-fork

# Sync organization forks using SSH
gh-pull-all --org myorg --pull-changes-to-fork --ssh

# Single-threaded fork sync for debugging
gh-pull-all --user myusername --pull-changes-to-fork --single-thread

🧪 Testing

• Added comprehensive CLI tests in test-pull-changes-to-fork-cli.mjs
• Tests validate CLI argument parsing, help output, and validation logic
• Integrated into existing test suite (test-all.mjs)
• All tests pass successfully

🛡️ Safety Features

• Uncommitted changes detection: Skips repositories with uncommitted changes
• Fork detection: Only operates on actual forks (skips regular repositories)
• Conflict handling: Reports merge conflicts without forcing destructive operations
• Branch fallback: Tries both main and master as default branches
• Remote management: Properly handles existing upstream remotes

🎨 User Experience

The new option integrates seamlessly with the existing status display system:

• Shows "Setting up upstream remote..." during remote configuration
• Displays "Fetching upstream changes..." during upstream fetch
• Updates with "Syncing with upstream/[branch]..." during merge
• Reports "Fork synchronized with upstream" on success
• Shows informative error messages for conflicts or failures

🔄 Workflow Integration

This feature works alongside existing options:

• Compatible with --ssh for SSH URL usage
• Works with --threads for concurrent processing
• Integrates with --live-updates for real-time status
• Follows same patterns as --pull-from-default and --switch-to-default

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This implementation provides a complete solution for fork synchronization while maintaining the tool's existing patterns and safety standards.