fix: Cloud missing_config should name config context add

[margaretjgu]

src/lib/cloud-client.ts tells users to elastic config set. That command is config current-context set. Cloud setup is elastic config context add <name> --cloud-url … --cloud-api-key ….

Same string on missing Cloud block and missing api_key.

This is Cloud-specific. #631 covers the generic no-file path.

Acceptance: unset Cloud on a context, run elastic cloud orgs list-organizations --json. error.message names elastic config context add and --cloud-api-key. No config set.