A test pins cutting a wave: an earlier wave that does not parse stops the cut, named, and nothing is written

cutWaveFor in src/actions/cut_wave.ts opens the draft waves/<next>.md pull; no test imports it.

Done, in test/cut-wave.test.ts:

[ ] On a repository with merged pulls since the last wave, an admin's cut opens one pull whose waves/<next>.md lists those pulls.
[ ] With an unparseable waves/*.md on the default branch, the cut answers an error that names that file, and no branch and no pull are created.
[ ] A non-admin's cut is refused and writes nothing.
[ ] No model is called (no network in the test).

Files: test/cut-wave.test.ts only. Suggested: scout.

A test pins cutting a wave: an earlier wave that does not parse stops the cut, named, and nothing is written #6827