Improve PyPI package metadata

[aniketmaurya]

Summary

• add a PyPI-focused README for the Python SDK and wire it through pyproject.toml
• update the package summary, keywords, and classifiers so the next PyPI release has useful project metadata
• add twine metadata checks to the Python publish workflow for both wheels and sdists

Validation

• live PyPI page currently shows: "The author of this package has not provided a project description"
• uvx maturin==1.14.1 sdist --manifest-path bindings/python/Cargo.toml --out /tmp/smolfs-pypi-dist-...
• uvx maturin==1.14.1 build --manifest-path bindings/python/Cargo.toml --interpreter python --out /tmp/smolfs-pypi-wheel-...
• uvx twine check
• inspected PKG-INFO and wheel METADATA; both include the new Markdown long description
• actionlint .github/workflows/*.yml
• ruby -e 'require "yaml"; Dir[".github/workflows/*.yml"].each { |f| YAML.load_file(f); puts "ok #{f}" }'
• git diff --check