[prompts] Add <environment_context> (#1869)

## Summary
Includes a new user message in the api payload which provides useful
environment context for the model, so it knows about things like the
current working directory and the sandbox.

## Testing
Updated unit tests

codex-rs/core/src/git_info.rs

@@ -9,7 +9,7 @@ use tokio::time::timeout;
 /// Timeout for git commands to prevent freezing on large repositories
 const GIT_COMMAND_TIMEOUT: TokioDuration = TokioDuration::from_secs(5);
 
-#[derive(Serialize, Deserialize, Clone)]
+#[derive(Serialize, Deserialize, Clone, Debug)]
 pub struct GitInfo {
     /// Current commit hash (SHA)
     #[serde(skip_serializing_if = "Option::is_none")]