Add missing export-ignore entries to .gitattributes

Add CLAUDE.md and .editorconfig to export-ignore so they are excluded
from the distributed archive. These files are only needed for
development, not for end users of the package.

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

Author: phpstan-bot

.gitattributes

@@ -5,6 +5,8 @@
 .github export-ignore
 tests export-ignore
 tmp export-ignore
+CLAUDE.md export-ignore
+.editorconfig export-ignore
 .gitattributes export-ignore
 .gitignore export-ignore
 Makefile export-ignore