feat(site): LLM-readable single-file bundle of the seven chapters

[pandazki]

Summary

• Build-time script bundles every mental-model chapter into one .txt file per language (/llms-mental-models-{en,zh}.txt) — meant to be pasted into an LLM chat or fed to an agent in one shot.
• Inline copy button sits next to the "For Non-Human Readers" / "致非人类读者" h2 on the homepage. Click → clipboard receives the bundle URL on the current origin; button flips to a vermilion "Copied" state for 1.6s.
• Chapter URLs in the bundle are path-only with the site base prefix (no host), so the file is portable across forks. The preamble explains how an LLM should reconstruct the host from where it fetched the bundle.
• UTF-8 BOM is prepended to each bundle so browsers render the file correctly even though GitHub Pages serves .txt without a charset= response header.
• Drive-by fix: site/astro.config.mjs site: was still pointing at a fork's host (nanxingw.github.io). Updated to panqiwei.github.io.

Test plan

• npm run build regenerates both bundles (prebuild hook → build:llm-bundle)
• npm run dev regenerates bundles (predev hook)
• npm run test:sync still passes (8/8)
• Bundles open as proper UTF-8 in the browser (BOM)
• Copy button writes the right URL to clipboard on both /zh/ and /en/
• Button label flips to "已复制" / "Copied" and back after 1.6s
• No baked-in host: grep -E '\.github\.io' site/public/llms-mental-models-*.txt returns nothing
• No UI changes outside the homepage (mm hub untouched)

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