docs: add Korean (ko-KR) README locale

[mvanhorn]

Summary

The README currently ships in English, Simplified Chinese, Japanese, and Vietnamese. This adds Korean so Korean-speaking developers can read the install steps, provider routing, and safety model in their own language. It is a translation-only change with no effect on the build or runtime.

README.ko-KR.md follows README.md section for section and keeps every command, flag, config key, provider and model ID, env var, file path, and code block literal so the instructions stay copy-pasteable. The provider support tiers match the English source exactly. The language selector in README.md gains a Korean entry, and the existing zh-CN, ja-JP, and vi READMEs cross-link back to it so navigation stays consistent across locales.

Because README.ko-KR.md carries the same version-pinned --tag vX.Y.Z install examples as the other locales, it is also added to the release version scripts (scripts/release/check-versions.sh and scripts/release/prepare-release.sh). Without that, the next release would bump the other READMEs and leave the Korean one with stale install commands.

This covers Korean only. Spanish, Brazilian Portuguese, and the website /ko route are separate follow-ups, which the issue explicitly allows by splitting per locale.

Refs #3093

Testing

Documentation-only change, so the cargo fmt/clippy/test workflow does not apply (no Rust code touched). I ran bash -n on both modified release scripts and python3 -m py_compile on the embedded version-bump block, and confirmed scripts/release/check-versions.sh still passes with the Korean README included.

Checklist

• Updated docs or comments as needed
• No tests needed (documentation-only change)
• No UI changes (documentation-only change)
• No harvested or co-authored credit in this PR