docs: add Simplified Chinese README

[whtis]

What

Add a Simplified Chinese translation of the README as README.zh-CN.md, and insert a small language switcher link at the top of README.md.

Why

Hyperframes already has a sizeable audience among Chinese-speaking developers and AI-agent users, but the README has been English-only. A native Chinese README lowers the barrier for first-time evaluation and helps the Quick Start instructions actually land.

The translation keeps the original structure 1:1 (every heading mirrored), preserves technical terms that the community uses unchanged (agent, skill, Frame Adapter, composition, seek, wall-clock, block, MP4, LFS, etc.), and only translates the prose around them. Tables, code blocks, badges, and external links are untouched so they stay easy to keep in sync.

How

• New file: README.zh-CN.md — full Simplified Chinese version of the README.
• README.md: one small <p align="center"> block added under the tagline pointing at the new file (mirrors the switcher already present at the top of README.zh-CN.md).

No other files touched. No tooling, build, or runtime impact.

Test plan

• Section headings in README.zh-CN.md match README.md line-for-line (verified with grep -nE '^##+ ' on both files).
• All relative links (LICENSE, CONTRIBUTING.md, packages/*, docs/guides/*, .codex-plugin/plugin.json, .claude-plugin/plugin.json, .cursor-plugin/plugin.json, assets/claude-code-icon-*.svg) preserved unchanged so they resolve from the repo root.
• External links (npm, Discord, hyperframes.heygen.com, vercel-labs/skills, etc.) preserved verbatim.
• Manually rendered locally — badges, language switcher, demo image, code blocks, and Packages/Skills tables render correctly.
• Manual testing
• Documentation updated (this is the doc change)

Happy to adjust terminology choices or split this differently if there's a preferred convention — for example, moving the file under docs/ or naming it README_zh-CN.md.

Drafted with AI assistance; verified by author before publishing.