Consolidate README docs and remove README.jp.md

[ryz310]

Summary

• Integrated the previously separate Japanese README content into README.md
• Removed README.jp.md to eliminate duplicate maintenance targets
• Updated AGENTS.md to remove README.jp.md references and align README maintenance rules

Purpose of this change

This repository previously maintained both README.md and README.jp.md because many users are Japanese.
Today, browser translation quality is high enough that keeping two full README files introduces more maintenance cost than benefit.
This change consolidates documentation into a single source of truth, reducing drift and documentation maintenance overhead.

Notes

• README.md now contains the practical information that existed only in README.jp.md
• Confirmed there are no remaining README.jp.md references in markdown documents

[qltysh]

Coverage Impact

This PR will not change total coverage.

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