Add Korean README translation (docs/README_ko.md)

[ohprettyhak]

Summary

Agent Reach already provides English (docs/README_en.md) and Japanese (docs/README_ja.md) README translations alongside the original Chinese README. Adding a Korean translation would make the project more accessible to Korean developers and AI agent users.

Motivation

• Korea has a growing AI agent developer community
• Many Korean developers use tools like Claude Code, Cursor, and OpenClaw
• A Korean README would lower the barrier to entry for Korean users
• Consistent with the project's existing i18n efforts (English + Japanese already available)

Proposed Approach

• Add docs/README_ko.md following the same structure and conventions as existing translations
• Update the language navigation links in README.md to include Korean
• Ensure natural, native-level Korean (not machine-translated)

References

• Existing English translation: docs/README_en.md
• Existing Japanese translation: docs/README_ja.md