fix: Ensure SSH key file has proper format for authentication

- Added proper handling of newline characters in SSH keys
- Checks for literal \n strings and converts to actual newlines if needed
- Ensures SSH key file ends with newline (required by SSH)
- Handles both direct newlines and escaped newlines from env vars
- Fixes "Load key: invalid format" error

The SSH key now writes correctly to temporary file with proper line breaks,
allowing SSH/SCP to authenticate without errors.