feat: add native SFTP directory operations and internalize SSH client

[inureyes]

Summary

• Internalized the async-ssh2-tokio library as src/ssh/tokio_client/ module
• Added native SFTP directory upload/download operations using russh-sftp
• Removed unused duplicate SFTP module implementation
• Improved performance with single-connection directory transfers

Major Changes

1. Internalized SSH Client Library

• Moved async-ssh2-tokio source code to src/ssh/tokio_client/
• Direct dependency on russh and russh-sftp instead of wrapper library
• Full control over SSH client implementation for future enhancements

2. Native Directory Operations

• upload_dir(): Recursively uploads directories using SFTP's native methods
• download_dir(): Recursively downloads directories using SFTP's readdir
• Automatic detection: uploads/downloads switch between file and directory methods based on path type
• Single SSH connection for entire directory transfer (previously: connection per file)

3. Code Cleanup

• Removed entire unused src/sftp/ module (8 files, ~2000 lines)
• Cleaned up duplicate implementations
• Maintained backward compatibility with existing CLI commands

Performance Improvements

• Before: Multiple SSH connections (one per file) for directory operations
• After: Single SSH connection with SFTP session for entire directory tree
• Significant speed improvement for directories with many files

User Impact

• ✅ No breaking changes - All existing commands work identically
• ✅ Same CLI interface (-r flag for recursive operations)
• ✅ Better performance for directory transfers
• ✅ More reliable handling of special characters in filenames

Test Results

• Unit tests: 24 passed ✅
• Integration tests: 31 passed ✅
• Doc tests: 2 passed ✅
• Total: 57 tests, all passing

Example Usage (unchanged)

# Upload directory recursively
bssh -c cluster upload -r ./local_dir /remote/path/

# Download directory recursively  
bssh -c cluster download -r /remote/dir ./local/