Add a Korean README (README_ko.md), covered by the localized-README drift guard

[cevheri]

The repo ships two localized READMEs, README_zh.md and README_ja.md (#317). Korean is not one of them.

This is smaller than it looks. README.md is 994 lines; the two translations are 304 and 291. They are deliberate abridgements, not full translations - they carry the pitch, the engine list, the install table and the quick start, and they drop the deep sections. Copy that scope rather than translating the whole file.

Fix.

1. Add README_ko.md, modelled on README_zh.md for structure and length.
2. Add <a href="README_ko.md">한국어</a> to the language switcher near the top of README.md, README_zh.md and README_ja.md (they each list the other languages).
3. Add "README_ko.md" to LOCALIZED in scripts/readme-check.mjs (line 34), so the drift guard covers it.

The guard decides two things for you, so review is objective. scripts/readme-check.mjs runs inside the required Lint, Typecheck and Build check and asserts:

• The engine name set is identical in all localized files and README.md. A translation that omits an engine is wrong, and so is one that invents an engine. Do not translate engine names - PostgreSQL stays PostgreSQL.
• Every command in a localized install table appears verbatim in README.md's. You may list fewer channels (both existing translations drop Chocolatey and the portable zip), but you may not paraphrase a command. A missing sudo or a dropped brew trust is exactly what this guard was written for after docs: add Simplified Chinese and Japanese READMEs #317's review found both.

Run bun run readme:check before you push.

Please only take this if you read and write Korean well. Raw machine-translation output is not what this is for, and a PR that reads as untouched machine output will be closed. Technical terms and product names are better left in English than translated approximately.

Conflict note. Step 2 touches one line in three files that every other language issue also touches. If another translation lands first, rebase - it is a one-line conflict.

Done when README_ko.md exists, the switcher in all four READMEs lists it, LOCALIZED includes it, and bun run readme:check passes.

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Curated for Hacktoberfest 2026. Comment to claim it before you start so two people do not work on the same language. A PR must reference this issue; see CONTRIBUTING.md. Repo rules that apply: run bun run test (never bare bun test), and the 100% line-coverage gate must stay green. Local bun run test also needs the helm binary on PATH for unrelated chart tests - see #570 if it bites.