Clone over ssh fails on first push #3

Fresh clone, first push to a branch nobody has pushed before, and the remote hangs up:

$ git push origin sl/my-branch
Enumerating objects: 12, done.
remote: error: cannot lock ref 'refs/heads/sl/my-branch'
To git@knot.tngl.boltless.dev:alice.pds.tngl.boltless.dev/core
 ! [remote rejected] sl/my-branch -> sl/my-branch (failed to update ref)

Second attempt succeeds, so it looks like a race in the hook rather than a permissions problem.