Add Caishen Readable built-in theme

[ChannelerH]

What changed

Adds Caishen Readable as a built-in Codex Styler theme. The theme uses public-safe original artwork from ChannelerH/codex-skin-packs, defines light and dark semantic variants, registers a Moss companion recommendation, and includes local preview/background assets.

Source/reference page with UTM attribution:
https://codex-theme-gallery.howardhua.chatgpt.site/themes/caishen-readable?utm_source=xuhuanstudio-codex-styler&utm_medium=github-pr&utm_campaign=caishen-readable

Evidence

• pnpm check
• pnpm lint
• pnpm typecheck
• pnpm test
• pnpm --filter @codex-styler/theme-core test
• pnpm --filter @codex-styler/theme-core typecheck
• Rust format, Clippy, and tests (no Rust changed)
• Screenshots or recording (theme preview asset included; no live app capture in this environment)
• English and Simplified Chinese checked
• Keyboard and reduced-motion behavior checked (no interaction changed)

Safety and compatibility

• No Codex installation files, signatures, or user data are modified
• Imported content remains data-only and local
• Unknown Codex versions still fall back safely; this only adds a built-in theme definition
• New assets have source and license entries in ASSET_LICENSES.md
• No private Codex screenshot, sidebar, task name, chat, or workspace path is included

Release note

Add the Caishen Readable built-in theme.

[chatgpt-codex-connector]

To use Codex here, create an environment for this repo.