Closes #ISSUE
Centralizes the references to the `ZED_STATELESS` env var into a single
location in a new crate named `zed_env_vars`
Release Notes:
- N/A *or* Added/Fixed/Improved ...
This breaks a transitive dependency of `agent` on UI crates. I've also
found and eliminated some dead code in assistant_context_editor.
Release Notes:
- N/A