ocs: add multilingual README support for Simplified Chinese, Traditional Chinese, and Japanese

[WangZan2001]

Summary

This PR adds multilingual README support to make AgentGuard more
accessible to a broader developer audience.

Changes

• Add a dedicated directory for localized README files: docs/i18n/
• Add Simplified Chinese translation: docs/i18n/README.zh-Hans.md
• Add Traditional Chinese translation: docs/i18n/README.zh-Hant.md
• Add Japanese translation: docs/i18n/README.ja.md
• Add language navigation links to the main README.md
• Update CONTRIBUTING.md with translation file naming and
  organization guidelines

Why

This project was shared with me through a community meetup, and while the English README is readable, it still creates friction for non-native English users during installation and initial evaluation.

For security tools like AgentGuard, reducing that friction matters. A localized README helps users understand the project faster, install it more confidently, and evaluate its capabilities without relying on browser translation tools or external plugins.

This PR aims to improve accessibility for users who are more comfortable reading Simplified Chinese, Traditional Chinese, or Japanese, while keeping the English README as the canonical source.

Notes

• English README.md remains the source of truth
• Localized files are grouped under docs/i18n/ to keep the
  repository root clean
• The first batch of supported languages includes:
  • Simplified Chinese
  • Traditional Chinese
  • Japanese