Add CLAUDE.md to .gitattributes export-ignore

CLAUDE.md is a development file for AI assistant context and should
not be included in the distributed archive.

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

Author: 2 people

.gitattributes

@@ -10,3 +10,4 @@ Makefile export-ignore
 phpstan.neon export-ignore
 phpstan-baseline.neon export-ignore
 phpunit.xml export-ignore
+CLAUDE.md export-ignore