Simplify CI into a single workflow

[G-Fourteen]

Summary

• replace the fragmented GitHub Actions pipeline with one unified job that builds the static site, runs tests, and handles GitHub Pages deployment
• ensure the deployment step only runs on successful pushes to main while still uploading artifacts and summaries for failed runs
• normalize the AI experience HTML file with a trailing newline to avoid formatting noise in future diffs

Testing

• pytest

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https://chatgpt.com/codex/tasks/task_b_69060c6259448329b8df8848e1a380de