docs: add Korean translation of AIP-4 spec

[minhchee]

Description

Adds Korean (ko) translation of AIP-4 (Agent Task Dispute Arbitration) specification.

Details

• Translated all 8 sections + 2 appendices from English to Korean
• Preserved all code blocks, JSON examples, endpoint paths, and RFC 2119 keywords (MUST, SHOULD, MAY) as-is
• Followed the same translation style as the existing Japanese translation (AIP-4.ja.md)
• All table content translated while preserving markdown table structure

Context

The specs/ directory currently has translations in de, es, fr, ja, pt-BR, pt, zh-CN but Korean (ko) is missing for all AIP specs. This PR adds the Korean translation for AIP-4.

Per the AIGEN Protocol README, translation contributions are eligible for AIGEN token rewards.