ci: gha-security のセキュリティチェック導入 + digest固定 + cooldown（パイロット）

[kaitoyama]

traPtitech/gha-security のパイロット導入です。Checkin は「bot 未導入のリポジトリにフルセットを入れる」確認役で、pin と cooldown の両方をこの PR だけで完結させています。

変更内容

ファイル	内容
.github/workflows/security-checks.yaml	PR 時のチェック: pin-support（未固定参照の修正を suggestion で提案）/ pin-check（未固定があれば fail）/ cooldown-check（公開から日が浅い依存バージョンの混入で fail。npm 7日・actions 3日）
docker-compose.yml	mariadb:11 に @sha256: digest を付与（gha-security の pin-docker による自動生成）
.github/dependabot.yml	actions の SHA / docker の digest / npm を週次で自動追従（cooldown 付き。セキュリティ修正は cooldown をバイパスして即時）
pnpm-workspace.yaml	minimumReleaseAge: 10080（7日）— 手元の pnpm add でも公開直後のバージョンが解決されなくなります（@traptitech/* は除外）

補足

• actions 3件は既に SHA 固定済みだったのでそのままです
• 緊急で新しいバージョンが必要な場合: PR に cooldown-override ラベル（CI 側）、pnpm add は一時的に --config.minimumReleaseAge=0（手元側）
• 各機能は独立してオン/オフできます（gha-security README 参照）。運用に合わなければジョブ削除や設定変更で調整してください

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📥 Commits

Reviewing files that changed from the base of the PR and between 770079a and f175447.

📒 Files selected for processing (4)

• .github/dependabot.yml
• .github/workflows/security-checks.yaml
• docker-compose.yml
• pnpm-workspace.yaml

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch gha-security-pilot

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