docs: maintain bilingual documentation and localize website navigation

[RyanLee-Dev]

Summary

Chinese website navigation previously opened English documentation. Add 44 authored Chinese mirrors under docs/zh/ and contracts/agents-api/zh/, plus a Chinese Harness catalog generated from the existing registrations. Docs, API, Architecture, the sidebar, homepage documentation cards and local search now use the selected language; language switching keeps the current document.

Require English and Chinese documentation to be maintained together in AGENTS.md. Build checks verify translation coverage and source freshness, preserve executable examples and inline identifiers, and compare document structure and heading anchors. English source documents and generated reference blocks remain authoritative.

This PR builds on the website styling changes in #389.

Validation

• Website builds at / and /OpenAgentCore/; all 22 website tests pass.
• Harness catalog generator check, name guard and whitespace checks pass.
• Browser smoke checks confirm Chinese navigation, same-document language switching and Chinese search results.
• Independent review completed; reported translation omissions and a protocol path typo were corrected and reviewed again.

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