Add Python type stubs and mypy to CI

[varunreddy]

Summary

Add mypy type checking to the CI pipeline and ensure all public APIs in src/skill_registry_rag/ have type annotations.

Acceptance Criteria

• Add mypy to pyproject.toml dev dependencies
• Add a mypy configuration section to pyproject.toml
• Add type annotations to all public functions in src/skill_registry_rag/
• Add a mypy check step to .github/workflows/ci.yml
• All existing tests still pass
• mypy passes with no errors