chore: add .mailmap to fold the Claude co-author identity (#107)

A few early commits (e.g. PR #100) carried a
`Co-Authored-By: Claude … <noreply@anthropic.com>` trailer, which
made GitHub's contributors graph list a spurious "claude"
contributor. .mailmap canonicalizes that identity to the maintainer
so shortlog / blame / the GitHub contributors graph no longer show
it. No history is rewritten; tags, releases (v0.9.0) and PyPI are
untouched. Future attribution is separately disabled via
~/.claude/settings.json (includeCoAuthoredBy: false).

Author: hyoshi

.mailmap

@@ -0,0 +1,9 @@
+# Canonicalize commit/co-author identities for git shortlog, blame, and
+# the GitHub contributors graph (all honor .mailmap).
+#
+# A few early commits carried a `Co-Authored-By: Claude … <noreply@anthropic.com>`
+# trailer (Claude Code attribution, since disabled). The human author is
+# the maintainer; fold that co-author identity into the canonical one so
+# tooling does not list a spurious "claude" contributor. No history is
+# rewritten — this only affects how identities are displayed.
+Hirokazu Yoshinaga <4027404+hyoshi@users.noreply.github.com> <noreply@anthropic.com>