fix: avoid exposing sensitive info in auth verification step (#3)

The whoami command outputs user details (email, collectives, etc.) into
the CI logs. Use --json output with jq to check only the authenticated
field, and print a generic success message instead.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Merge pull request #1 from systeminit/initial-action

feat: add setup-swamp GitHub Action