* docs: add a Korean README translation
* docs(changelog): reference #6909 for the Korean README
* docs(readme-ko): restore blank lines before headings and sync with README.md
Six headings sat directly under the preceding paragraph or code fence where
README.md keeps a blank line. Also translate the README.md additions from
#6898 (Codex tool_choice), #6886 (mixed-provider citation cleanup), and #6701
(authenticated HTML preview resources).
