Refactor cooperbench adapter to match harbor adapters init format (#1419)

- Move to src/cooperbench/{adapter,main}.py uv-package layout
- Rename CooperBenchToHarbor → Adapter (alias kept for back-compat)
- Rename template/ → task-template/
- Move subsets/ and fixes/ into src/cooperbench/
- Add pyproject.toml, main.py CLI entry point
- Delete run_adapter.py (replaced by `python -m cooperbench.main`)
- Update README commands

Verified: byte-identical task output vs pre-refactor adapter.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>