docs(ko): polish Korean phrasing in acp, agents, config, and custom-tools docs

[Seungjun0906]

Fixes #13440

CI note

• The failing CI job appears unrelated to this docs-only change.
• The failure occurs in Windows e2e with dependency/runtime errors (e.g. Cannot find package 'zod' from .opencode/node_modules/@opencode-ai/plugin/dist/tool.js), while this PR only edits Korean MDX documentation files.

What does this PR do?

This PR improves wording quality in four Korean docs pages:

• ko/acp.mdx
• ko/agents.mdx
• ko/config.mdx
• ko/custom-tools.mdx

These pages had several literal translations that made parts of the docs awkward to read.

I rewrote phrasing to sound natural in Korean while preserving technical meaning and structure.

I only changed documentation wording and consistency (tone/terminology), and kept commands, links, code blocks, and headings intact.

How did you verify your code works?

• Reviewed each file diff against the English source docs.
• Checked MDX structure integrity (frontmatter, headings, code blocks, links).
• Confirmed this PR is docs-only and does not change runtime behavior.