docs(readme): add Chinese translation (zh-CN)

[whtis]

What

Adds a Simplified Chinese translation of the README as README.zh-CN.md, and adds a small language switcher at the top of the English README so the two stay discoverable from each other.

Why

CodeGraph has growing adoption among Chinese-speaking developers using Claude Code, but the README is English-only. A native zh-CN version lowers the barrier to first install and to understanding the benchmark numbers, framework-routing table, and global-instructions block (which is the part most likely to be misread under a quick machine-translation).

How tested

• Side-by-side diff against the English source to confirm headings, anchors, code fences, badge URLs, and shields stay byte-identical.
• Inline links (./README.md, ./README.zh-CN.md, tree-sitter.github.io, issues page, MCP config paths) checked manually.
• Tables, <details> collapses, and the ASCII architecture diagram render correctly in GitHub preview.
• No source code changed; only README.md (+2 lines for the switcher) and the new translation.

Notes

• Technical terms kept in English where they are proper nouns or identifiers (tree-sitter, FTS5, MCP, Explore agent, Session.request(), all CLI flags and tool names). The goal was a translation a Chinese engineer would actually read, not one that hides the API surface.
• Two-file diff, no behavior change. Happy to split the switcher line into a separate PR if you'd prefer to merge those independently.

Drafted with AI assistance; verified by author before publishing.