chore: update CODEOWNERS to @KikyoNanakusa (#253)

## Why

CODEOWNERS の `@araaki12345` を現室長である
`@KikyoNanakusa`（https://github.com/KikyoNanakusa）に差し替える。

## What

- `CODEOWNERS` の1行を更新: `@araaki12345` → `@KikyoNanakusa`
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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: KinjiKawaguchi

CODEOWNERS

@@ -1 +1 @@
-* @KinjiKawaguchi @araaki12345
+* @KinjiKawaguchi @KikyoNanakusa