fix: make NPM_AUTH_TOKEN optional in .yarnrc.yml for local development

Use ${NPM_AUTH_TOKEN-} syntax to allow empty default when env var is not set.
This enables local yarn install without the token while still working in CI/CD.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: jkebinger

.yarnrc.yml

@@ -1,3 +1,3 @@
 nodeLinker: node-modules
 npmRegistryServer: 'https://registry.npmjs.org'
-npmAuthToken: '${NPM_AUTH_TOKEN}'
+npmAuthToken: '${NPM_AUTH_TOKEN-}'