clean up repo: add license, rewrite readme, remove dead terraform
- add MIT license
- rewrite README to lead with the pipeline, not the infrastructure
- remove unused prefect/ terraform directory (superseded by scripts/patch_work_pool.py)
- note ANTHROPIC_API_KEY in .env.example (it's a prefect secret block)
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>