feat(doi): add DOI-based path access for commands

Support accessing repositories using DOI strings with optional subpaths
across ls, download, metadata, and file-metadata commands.

- Implement GET v3/doi/{id}/ API model and client calls
- Parse and resolve DOI paths into respective folder and files
- Extract common folder and file resolution logic to shared helpers
- Update README with example DOI-based shell commands

mdrsclient/commands/mkdir.py

@@ -23,7 +23,7 @@ class MkdirCommand(BaseCommand):
 
     @classmethod
     def mkdir(cls, remote_path: str) -> None:
-        (remote, laboratory_name, r_path) = cls._parse_remote_host_with_path(remote_path)
+        remote, laboratory_name, r_path = cls._parse_remote_host_with_path(remote_path)
         r_path = r_path.rstrip("/")
         r_dirname = os.path.dirname(r_path)
         r_basename = os.path.basename(r_path)