chore: add @vitest/coverage-v8 for CI coverage reporting

CIの--coverageフラグ実行に必要な依存を追加。

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: berlysia

package.json

@@ -69,6 +69,7 @@
     "@types/seed-random": "2.2.4",
     "@types/shuffle-array": "1.0.5",
     "@types/yargs": "17.0.33",
+    "@vitest/coverage-v8": "3.2.4",
     "conventional-changelog-cli": "2.2.2",
     "eslint": "9.36.0",
     "lint-staged": "15.5.2",