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/tui/src/app_command.rs

@@ -150,6 +150,7 @@ impl AppCommand {
     ) -> Self {
         Self(Op::UserTurn {
             items,
+            environments: None,
             cwd,
             approval_policy,
             approvals_reviewer: None,
@@ -296,6 +297,7 @@ impl AppCommand {
                 final_output_json_schema,
                 collaboration_mode,
                 personality,
+                environments: _,
             } => AppCommandView::UserTurn {
                 items,
                 cwd,

codex-rs/tui/src/app_server_session.rs

@@ -532,6 +532,7 @@ impl AppServerSession {
                     thread_id: thread_id.to_string(),
                     input: items.into_iter().map(Into::into).collect(),
                     responsesapi_client_metadata: None,
+                    environments: None,
                     cwd: Some(cwd),
                     approval_policy: Some(approval_policy.into()),
                     approvals_reviewer: Some(approvals_reviewer.into()),