♻️ 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".

CHANGELOG.md

@@ -8,6 +8,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 - require a short summary under `[Unreleased]` in the changelog before a branch is finished
+- let `create:branch` start from a `main` that is ahead of its upstream, so a finished merge no longer blocks the next branch until it is pushed
 
 ## [0.1.6] - 2026-09-15