Add multilingual README hero

[denial123789]

Summary

• add a lightweight 3:1 hero illustrating the secure CLI/MCP bridge and its tool categories
• show the same language-neutral visual in all eight READMEs
• use an absolute GitHub asset URL so npm-rendered READMEs can display it
• keep the asset under .github/assets so it does not enter the npm tarball

The image contains no text, logos, people, or third-party marks. It was generated for this repository and compressed from a 1.4 MB PNG to a 78 KB WebP.

Verification

• visually inspected the final 2172×724 WebP
• npm run build
• npm run audit:package
• git diff --check
• confirmed all eight READMEs reference the asset