Add Caishen Readable free theme

[ChannelerH]

Summary

• adds caishen-readable, a light Caishen-inspired free theme pack
• includes responsive backgrounds, HTML/CSS preview screenshots, preview evidence, visual tokens, README, and asset license
• keeps previews public-safe: fixed demo content only, no private Codex screenshots or workspace data

Source/preview reference: https://codex-theme-gallery.howardhua.chatgpt.site/themes/caishen-readable?utm_source=get-codex-theme-pr&utm_medium=github&utm_campaign=caishen-readable

Validation

• node packages/theme-cli/bin/get-codex-theme.mjs validate themes/free/caishen-readable --strict-assets
• npm run test:themes

[ChannelerH]

Validation refresh for maintainers/users:

• node packages/theme-cli/bin/get-codex-theme.mjs validate themes/free/caishen-readable --strict-assets -> passes
• npm run test:themes -> all free theme manifests pass strict asset validation, including caishen-readable
• git diff --check -> clean
• Live source page returns HTTP 200: https://codex-theme-gallery.howardhua.chatgpt.site/themes/caishen-readable?utm_source=get-codex-theme-pr&utm_medium=github&utm_campaign=caishen-readable
• Direct source ZIP returns HTTP 200: https://codex-theme-gallery.howardhua.chatgpt.site/theme-packs/caishen-readable.zip

This keeps the PR as a real free theme pack for the get-codex-theme install/apply flow, not just an external gallery link.