Create CODE_OF_CONDUCT.md with Contributor Covenant v2.1

[Copilot]

Adds a Code of Conduct based on Contributor Covenant v2.1 to establish community standards for participation.

Changes

• Expected behaviors: Empathy, respect for differing viewpoints, constructive feedback
• Unacceptable behaviors: Harassment, trolling, doxxing, inappropriate conduct
• Reporting: Direct link to GitHub Issues for incident reporting
• Enforcement: Four-tier escalation (correction → warning → temp ban → permanent ban)

Document is in Japanese to match project language preferences.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[中優先] CODE_OF_CONDUCT.md の作成</issue_title>
<issue_description>## 概要
行動規範 CODE_OF_CONDUCT.md を作成する。

背景

• PM レビュアーが指摘
• 心理的安全性の高いチーム環境構築
• OSS コミュニティ参加時の必須要件

含めるべき内容

• Contributor Covenant の採用
• 期待される行動
• 許容されない行動
• 報告方法

優先度

🟢 中優先（PM独自）</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [中優先] CODE_OF_CONDUCT.md の作成 #23

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