#3608 Registry drops contentMetadata.extensions (method grafts) at confirm-push

The CLI sends contentMetadata.extensions (ExtractedExtension: fileName, extendsType, methods, resources — methods grafted onto a foreign type) on every push. validateContentMetadata in lib/domain/extension/extension-content.ts has no entry for it, so the registry discards it: it is never stored on the version, never shown on the extension page, and never returned from GET /api/v1/extensions//latest.

Found while fixing swamp-club#3592 (webhooks dropped the same way). After that fix, confirm-push warns about a non-empty unrecognised key, so pushes of extensions that graft methods will now print 'contentMetadata.extensions is not indexed by the registry and not stored' until this kind is indexed or deliberately ignored.

Decide whether grafts should be a registry content kind (validator, contentTypes, view, detail page) or an explicitly ignored key.