Add CLAUDE.md to export-ignore in .gitattributes

CLAUDE.md is a development-only file and should not be included
in the distributed archive.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: 2 people

.gitattributes

@@ -7,4 +7,5 @@ tmp export-ignore
 .gitignore export-ignore
 Makefile export-ignore
 phpstan.neon export-ignore
+CLAUDE.md export-ignore
 phpunit.xml export-ignore