gh stack rebase conflicts when the bottom layer was squash-merged

If gh stack rebase conflicts on the very first layer, check whether that layer's commits were already squash-merged into trunk under another PR: the rebase replays history trunk already has in squashed form. Check with git log --graph whether the next layer's parent is trunk itself. The fix is to drop the layer, not resolve conflicts: gh stack rebase --abort (switch to a branch where stray build outputs are ignored, or it refuses), gh stack unstack, gh stack init <remaining branches>, then gh stack submit --auto. That relinks the existing PRs and retargets the new bottom PR to trunk. Close the redundant PR, pointing to the squash merge.