Publish & install components in the 21st team library

Use this skill in two directions: publishing a component for the team, and installing one the team already shared.

Pre-flight (always)

1. Check auth: cat ~/.an/credentials exists, or API_KEY_21ST env is set. If neither — tell the user to run npx @21st-sdk/cli login once. Don't try to log them in yourself.
2. The CLI is @21st-dev/registry. Don't reinvent — use it.

Publishing a component

Decide visibility — default to team