feat: add SFTP file transfer with upload/download commands

[inureyes]

Summary

This PR implements comprehensive SFTP file transfer functionality for bssh, enabling secure file upload and download operations across multiple cluster nodes in parallel.

Changes

Core SFTP Implementation

• ✅ Implemented SFTP upload functionality using async-ssh2-tokio
• ✅ Implemented SFTP download functionality
• ✅ Added parallel file transfer across multiple nodes
• ✅ Renamed copy command to upload for better clarity
• ✅ Added new download command for retrieving files

Glob Pattern Support (NEW)

• ✅ Added wildcard/glob pattern support for batch operations
• ✅ Local glob expansion for upload (e.g., *.txt, logs/*.log)
• ✅ Remote glob expansion for download via SSH ls command
• ✅ Automatic directory detection for multi-file uploads
• ✅ Human-readable file size formatting in output

Progress Tracking

• ✅ Real-time progress indicators with spinners
• ✅ Status messages showing connection, transfer, and completion states
• ✅ Per-node success/failure reporting

Documentation

• ✅ Updated manpage with upload/download command documentation
• ✅ Added glob pattern examples and usage instructions
• ✅ Included SFTP requirements and configuration notes

Usage Examples

Upload files with glob patterns

# Upload all log files
bssh -c production upload "*.log" /var/logs/

# Upload specific pattern
bssh -c staging upload "config/*.yaml" /etc/app/

Download files with glob patterns

# Download all conf files
bssh -c production download "/etc/*.conf" ./backups/

# Download application logs
bssh -c webservers download "/var/log/app*.log" ./collected_logs/

Single file operations

# Upload single file
bssh -c production upload deploy.tar.gz /tmp/

# Download single file
bssh -c production download /etc/passwd ./downloads/

Technical Details

• Uses async-ssh2-tokio's native SFTP support for secure transfers
• Implements proper error handling with detailed error messages
• Maintains unique file naming for downloads (prefixed with hostname)
• Supports both individual and batch file operations
• Preserves async/parallel execution model

Testing

• Added test_glob.sh script for glob pattern testing
• Manual testing completed on localhost
• All existing tests pass

Checklist

• Code follows project style guidelines
• Documentation updated (manpage)
• Tests pass
• Glob pattern support implemented
• Error handling implemented
• Progress indicators working

This completes the SFTP implementation milestone from TODO.md.