[context] Store context messages in rollouts (#2243)

## Summary
Currently, we use request-time logic to determine the user_instructions
and environment_context messages. This means that neither of these
values can change over time as conversations go on. We want to add in
additional details here, so we're migrating these to save these messages
to the rollout file instead. This is simpler for the client, and allows
us to append additional environment_context messages to each turn if we
want

## Testing
- [x] Integration test coverage
- [x] Tested locally with a few turns, confirmed model could reference
environment context and cached token metrics were reasonably high

codex-rs/core/src/lib.rs

@@ -17,6 +17,7 @@ pub mod config;
 pub mod config_profile;
 pub mod config_types;
 mod conversation_history;
+mod environment_context;
 pub mod error;
 pub mod exec;
 pub mod exec_env;