src/cli(refactor[add]): simplify preserve_cwd_label to explicit-only

why: The path is None heuristic dates back to the deprecated name+URL flow; path-first adds should only create ./ when explicitly requested.
what:
- Remove workspace_root_path is None branch that implied path is None
- Set preserve_cwd_label directly from explicit_dot flag
- Fix variable redefinition in _prepare_no_merge_items

refs: Legacy name+URL flow deprecation

Author: tony