CI: add manuscript build & figure generation

[wilcompute]

Add CI job to build manuscript and generate figures.

• Add a GitHub Actions workflow that installs TeXlive/pandoc (or uses a pre-built container) and builds paper/PAPER_DRAFT.md into PDF/HTML.
• Add a job step that runs figure generation scripts (from paper/figures/ or scripts/) so figures are always up-to-date and artifacts are uploaded.
• Document the build in CONTRIBUTING.md and add a badge to README.