* docs(readme): replace star-history.com with self-generated chart
The star-history.com SVG is a third-party dependency that can rate
limit or go down. Replace it with a GitHub Action that fetches
stargazers via the REST API and renders an SVG with matplotlib,
committing note/star_history.svg weekly. The README references the
committed file directly, so the chart has no runtime dependency on
any external service.
* ci(star-history): run daily instead of weekly
