A config-PR deploy was one opaque node that fetched, verified, merged,
built and compensated. That shape made three things impossible: the
nix-heavy phases could not take the meta window without the cheap ones
holding it too, a crash mid-build left no node to run the rollback, and
the dashboard could only ever show "deploying" for the whole thing.
Replace it with a `DeployWindow` group root over `MergeVerify ->
DeployApply` (AfterOk) plus a `DeployTail` hanging off the apply with
AfterAny, so the tail runs whether the apply succeeded, failed, or was
cancel-cascaded by a failing verify.
bdf15168db