Every push queues a run on the shared runner, and a branch pushed again
left its older run queued for a head nobody needed. The workflow now puts
each branch's runs in one concurrency group with cancel-in-progress, so a
new push cancels that branch's older run, queued or running. The group is
keyed on the branch, so pushes to other branches never cancel runs on
`next` or `main`; a push to `next` itself does cancel the older `next` run.
The checkout step no longer writes the token into `.git/config`;
`script/cibuild` does not need it.
Model: opus-5-5
sneak