xlm-scaffold: add --no-clobber to avoid overwriting root README/setup.py

[dhruvdcoder]

Problem

When adding an external model to an existing multi-model repo, xlm-scaffold <model_name> writes root-level README.md and setup.py (under --output-dir) and can overwrite project files. xlm_models.json already merges safely when present, but README/setup.py do not.

Current workaround used by the xlm-new-model-scaffold skill:

xlm-scaffold <model_name> --output-dir /tmp/xlm_scaffold_<model_name> --no-xlm-models
cp -R /tmp/xlm_scaffold_<model_name>/<model_name> .
# manually merge xlm_models.json / README / setup.py as needed

Proposal

Add xlm-scaffold --no-clobber (name flexible) that:

1. Skips writing root README.md / setup.py when those paths already exist.
2. Optionally still creates them when missing (greenfield stays convenient).
3. Keeps the existing merge behavior for xlm_models.json (or documents using --no-xlm-models + hand edit).

Related: src/xlm/commands/scaffold_model.py (generate_setup_py, README generation, --output-dir, --no-xlm-models).