Add screenshot refresh workflow

[Yassinbrine]

Addresses #23

Summary

• add a reusable Matplotlib screenshot runner that forces the Agg backend and saves the active figure
• add a monthly/manual GitHub Actions workflow that renders local-data charts and opens a refresh PR through peter-evans/create-pull-request
• keep the networked corporate-travel renderer behind an explicit manual input so scheduled runs avoid routine external scraping

Validation

• python -m py_compile scripts/render_screenshot.py
• git diff --check

I did not run full chart rendering locally because this environment does not have the repository's Python dependencies installed.

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