Fix startup errors: remove dead model imports, add missing pyyaml

- musehub/models/__init__.py: remove imports of requests/responses/tools modules that don't exist (leftover mono-repo LLM API stubs) - requirements.txt + pyproject.toml: add pyyaml>=6.0.0 (used in repos.py but was never declared as a dependency) - pyproject.toml: remove stale gradio_client mypy ignore override