Build and operate Domino UI Extensions (mount Domino Apps in the shell with page context). Covers extension_manifest.json, official installer vs manual Extension API, publish and identity requirements, auth and host splits, and beta REST routes. Use when creating an Extension, wiring mount points, publishing an Extension App from a project, or automating Extension install lifecycle. Not for generic App deployment alone (see domino-apps) or SPA scaffolding (see domino-ui-bootstrap).