Remove yarn, use npm exclusively

Replace all yarn references with npm across README, CLAUDE.md,
CHANGELOG, .gitignore, and CI workflow. Remove yarn.lock.

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

.gitignore

@@ -15,8 +15,8 @@ vendor/
 .DS_Store
 Thumbs.db
 
-# Yarn
-yarn-error.log
+# npm
+npm-debug.log
 
 # Backup files
 *.bak