docs: add Korean translation of ARCHITECTURE.md

[minhchee]

Description

Adds ARCHITECTURE.ko.md — the complete Korean (ko) translation of the root document ARCHITECTURE.md.

Details

• Translated all sections while preserving every code block, URL, endpoint, JSON snippet, and table verbatim.
• Pure documentation — no behavioral change, safe for CI.

Verification

• Markdown code fences balanced.
• Line endings: LF only (no CRLF).

Context

Extends the Korean documentation set (README.ko.md, AIGEN_PROTOCOL.ko.md, and now ARCHITECTURE.md). Root documentation previously had zero translations while specs/ carried 7 languages.