Allow configuring a base Datasette instance URL and API token via
environment variables, so users can run commands like
`dclient query data "select ..."` without specifying a full URL each time.
DATASETTE_URL is combined with the argument as a path segment.
DATASETTE_TOKEN is used as a lowest-priority fallback for auth.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
