♻️ Let create:branch start from an ahead main

`create:finish` leaves the merge local until `create:release` pushes it, so
`main` is routinely ahead of its upstream between a merge and a release. The
branch front door demanded an exact match and refused until it was pushed,
forcing a manual `git push` that the model deliberately keeps out of it.

Reject only a `main` that is behind its upstream — matching `create:finish`,
which already tolerates being ahead — and record why pushing from `finish` was
rejected instead. The push stays with `create:release`, so the merge remains
reviewable locally, and the known issue about the deadlock is gone.

Resolves: backlog task "Resolve the finish/push tension".

backlog.tasks

@@ -29,7 +29,7 @@ Documentation:
 ☐ Validate code fences in Markdown (start with README.md) — compile the TypeScript examples against `src/` so the docs cannot drift from the API
 
 Workflow:
-☐ Resolve the finish/push tension: `create:finish` leaves `main` ahead of its upstream while `create:branch` refuses until `main` matches upstream — decide whether `finish` should push or `branch` should compare only `BEHIND` (see development/workflow.md)
+✔ Resolve the finish/push tension: `create:finish` leaves `main` ahead of its upstream while `create:branch` refuses until `main` matches upstream — decide whether `finish` should push or `branch` should compare only `BEHIND` (see development/workflow.md) @done
 
 Maintenance:
 ☐ Serve CI coverage over a tiny self-hosted webserver (replace the zip artifact) @low