[Claude] Add Dependabot configuration

[mpyw]

目的

依存関係の自動更新を Dependabot で整備する。

方針

• メジャー更新: 個別 PR
• マイナー / パッチ更新: グループで 1 つの PR にまとめる（update-types: [minor, patch]）
• サプライチェーン対策: cooldown: default-days: 7（新バージョン公開後 7 日待ってから bump PR を出す＝実質 minimum-release-age）
• 対象エコシステム: gomod + github-actions

影響

.github/dependabot.yml の追加のみ。既存コードへの影響なし。

🤖 This PR was automatically created by Claude

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (a280232) to head (1925062).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main        #6   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines          108       108           
=========================================
  Hits           108       108           

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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