Fallback descriptor for elements not defined by code or plugins.
This descriptor is intentionally conservative. It lets a caller classify an
unregistered agent:* component as a known safety shape, but it does not
grant semantic mutation rights. Until a plugin registers a specific
descriptor, unknown element content is operator-authoritative and merge-only.
__unknown__ is an internal sentinel name, not a document marker. Do not
reserve agent:unknown; a plugin may define that component normally later.