Add CLAUDE.md to export-ignore in .gitattributes

Exclude CLAUDE.md from distribution archives since it is only needed
for development and AI assistant guidance, not by end users.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

Author: 2 people

.gitattributes

@@ -9,3 +9,4 @@
 /phpstan.neon export-ignore
 /phpstan-baseline.neon export-ignore
 /phpunit.xml export-ignore
+/CLAUDE.md export-ignore