Add missing export-ignore rules to .gitattributes

Add export-ignore for .editorconfig, phpstan-baseline.neon, and
CLAUDE.md so they are excluded from the distributed archive. These
are development-only files not needed by end users.

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

Author: 2 people

.gitattributes

@@ -5,8 +5,11 @@
 .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
 phpstan.neon export-ignore
+phpstan-baseline.neon export-ignore
 phpunit.xml export-ignore