docs: add self-updating star history

[avs-io]

What changed

• add a Star History section near the bottom of the README
• seed light and dark SVG charts plus a PNG fallback from live CodeBurn stargazer data
• add a daily/manual workflow that refreshes the assets using the repository-scoped GITHUB_TOKEN
• pin the third-party chart action to an immutable commit because the workflow has contents: write

Why

The legacy hosted Star History SVG now returns a GitHub-access-restriction placeholder. Generating the chart inside this repository keeps the README working without placing a reusable PAT in a public URL.

Validation

• generated the light and dark charts from live getagentseal/codeburn stargazer data (9,250 stars at validation time)
• verified both SVGs with xmllint --noout
• verified the PNG is 1920 x 1280 RGBA
• reran the action render script against the seeded assets; it reported changed=false
• confirmed generated assets contain neither a GitHub token nor the restricted-data placeholder
• parsed the workflow YAML successfully

Operational note

The repository currently defaults workflow permissions to read, but this workflow explicitly requests contents: write. If organization policy overrides that request, the first manual run will expose it and the workflow can be adjusted to open a PR instead of pushing directly.