glas_harness/reins/rein_openweights.py implements the Rein ABC by
shelling out to `rein-openweights run --task-file ... --no-hub
[--model ...]`, mirroring reins/rein_aharness.py exactly. Factored the
duplicated git_head/write_task_file logic into reins/_shared.py, used
by both adapters now. registry/reins/rein-openweights.yaml flipped to
status: implemented. 8 new tests (mocked subprocess), 18/18 passing.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
