Add .gitignore to prevent OS, editor, and build artifacts from being tracked

[Copilot]

Creates .gitignore to prevent repository pollution from developer environment artifacts.

Covered patterns:

• OS files: .DS_Store, Thumbs.db, system metadata
• IDEs: .idea/, .vscode/* (preserves shared config), Vim, Eclipse, Sublime
• Node.js: node_modules/, package manager logs
• Build outputs: dist/, build/, .next/, .nuxt/, framework caches
• Environment: .env* variants
• Test/logs: coverage/, *.log

VS Code shared settings (settings.json, tasks.json, launch.json, extensions.json) remain tracked for team consistency.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[最優先] .gitignore の作成</issue_title>
<issue_description>## 概要
Git管理対象外ファイルを定義する .gitignore を作成する。

背景

• 全てのレビュアー（DevOps/Frontend/PM）が必須として指摘
• OS固有ファイルやエディタ設定がリポジトリを汚染するのを防止

含めるべき内容

• OS固有ファイル（.DS_Store, Thumbs.db）
• エディタ/IDE設定（.idea/, .vscode/（一部を除く））
• node_modules/
• ビルド成果物
• 環境設定ファイル（.env等）

優先度

🔴 最優先（全員一致）</issue_description>

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

• Fixes [最優先] .gitignore の作成 #2

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