Implementation environment

Ideally something that:
* Can work locally, or be deployed to a cloud instance
* Coding harness has a simple path to running acceptance tests on different stack versions (likely excludes Github Actions/AW)
* Is sufficiently observable that we're not shooting ourselves in the foot
* Is model provider agnostic

Top of mind, this means building on top of Gas City, though we should do some proper evaluation there.

Start by evaluating Gas City against the stated requirements: local or cloud deployment, a simple path to acceptance tests across stack versions, strong observability, and model-provider agnosticism. Define what the selected implementation environment must provide and document how completion will be judged.

Written by the indexing model from the issue text.