Add turn-scoped environment selections (#18416)

## Summary
- add experimental turn/start.environments params for per-turn
environment id + cwd selections
- pass selections through core protocol ops and resolve them with
EnvironmentManager before TurnContext creation
- treat omitted selections as default behavior, empty selections as no
environment, and non-empty selections as first environment/cwd as the
turn primary

## Testing
- ran `just fmt`
- ran `just write-app-server-schema`
- not run: unit tests for this stacked PR

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core/tests/suite/skills.rs

@@ -99,6 +99,7 @@ async fn user_turn_includes_skill_instructions() -> Result<()> {
     let session_model = test.session_configured.model.clone();
     test.codex
         .submit(Op::UserTurn {
+            environments: None,
             items: vec![
                 UserInput::Text {
                     text: "please use $demo".to_string(),