Render a domain before adding it as an asset on a monitor with a capture reference. A preview exists only for a planned add_asset: it needs the monitor, passes the same checks add_asset would (valid domain, plan slot free, not already watched) and its confirm records that the user asked for the add. Returns the screenshot as image content so it can be shown to the user, plus a capture_token bound to that domain with its expiry in expires_at (a few minutes). The image is for the user to approve; the token then goes to add_asset as reference.capture_token, and a new preview_domain call retries the render. captured marks a usable capture: a favicon-only render has a token but no image, while a failed or blocked render returns the outcome with neither. Requires the mcp:write scope, since it only makes sense when setting up monitoring.