Add sticky thread environments

Carry thread-level environment selections through app-server, core turn context, unified exec, and environment manager plumbing. Update generated schema and existing tests for the new environment fields.

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

codex-rs/cli/src/main.rs

@@ -424,7 +424,7 @@ struct AppServerCommand {
 
 #[derive(Debug, Parser)]
 struct ExecServerCommand {
-    /// Transport endpoint URL. Supported values: `ws://IP:PORT` (default).
+    /// Transport endpoint URL. Supported values: `ws://IP:PORT` (default) and `stdio://`.
     #[arg(
         long = "listen",
         value_name = "URL",