chore: 新增 .mailmap，將 yao@xihui.io 對應到 ritim 帳號身分

修正 contributor 歸屬：先前所有 commit 使用未連結 GitHub 帳號的 email，
導致本 repo 在 ritim 個人檔案的釘選清單中不可見。

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ritim

.mailmap

@@ -0,0 +1,4 @@
+# Maps commit identities to the canonical GitHub account (ritim).
+# Commits authored with yao@xihui.io are the same person as ritim;
+# this links them for `git shortlog`/`git log` and GitHub's contributor view.
+Yao <5123299+ritim@users.noreply.github.com> <yao@xihui.io>