[codex] Add GitHub Pages PDF reader

[ziyu111777]

Summary

• Add a root index.html for GitHub Pages so readers have a stable online PDF reading entry instead of relying on the GitHub blob preview.
• Add visible open/download/Markdown fallback links for browsers that do not render the embedded PDF.
• Update the README top section with online reading, direct PDF download, and a note about GitHub large-PDF preview failures.

Root cause

GitHub blob pages can fail to render large PDFs and show misleading messages such as "无法渲染代码块". The PDF itself is valid; the fix avoids using the blob page as the primary reading surface.

Validation

• git diff --check
• Parsed index.html with Python html.parser
• Served the repo locally with python3 -m http.server 8765
• Confirmed / returns 200 text/html
• Confirmed /Codex%E6%A9%99%E7%9A%AE%E4%B9%A6.pdf returns 200 application/pdf
• Captured a local Chrome headless screenshot to verify the page header/actions render`