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

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 `한국어` 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 #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.

Use README_zh.md as the structural and scope model, then update the language switchers in README.md, README_zh.md, and README_ja.md. Add README_ko.md and include it in LOCALIZED in scripts/readme-check.mjs; run bun run readme:check, and confirm the required test and coverage gates remain green.

Written by the indexing model from the issue text.