Explain that bare `rea doctor` audits every integration, map common tasks
to `--provider`, `--client`, and `--skill` scopes, and describe how
`scope_checks`, `informational_checks`, `healthy`, and
`environment_healthy` relate. Document the typed ambiguous-provider
error and how to choose an engine once without implying a fallback.
Fixes#726
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>