Add CLAUDE.md to .gitattributes export-ignore

CLAUDE.md contains project instructions for AI assistants and is not
needed in the distributed Composer archive. Add it to the export-ignore
list alongside other development-only files.

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

Author: 2 people

.gitattributes

@@ -5,6 +5,7 @@ e2e export-ignore
 tmp export-ignore
 .gitattributes export-ignore
 .gitignore export-ignore
+CLAUDE.md export-ignore
 Makefile export-ignore
 phpstan.neon export-ignore
 phpunit.xml export-ignore