docs: keep README images visible on npm

[Grivn]

Summary

• serve the English and Chinese README images from absolute GitHub URLs so npm can render them without shipping documentation media
• keep the README poster, animated demo, and conversation screenshots while preserving the lean runtime package
• reject future relative README image URLs during package verification

Verification

• all four referenced image assets return HTTP 200
• pnpm run verify:package
• package remains at 49 files and approximately 192 KB packed
• git diff --check