tool: task types should be closed

[rogpeppe]

commit 76252f4

I misspelled $after as after and nothing complained. It would have been very helpful if it had (i.e. task types should be closed definitions to avoid misspelled field names).

[cueckoo]

This issue has been migrated to cue-lang/cue#327.

For more details about CUE's migration to a new home, please see cue-lang/cue#1078.