Add sticky thread environment selections

Allow thread/start to configure sticky environment selections that are used by turns when no per-turn override is supplied. Per-turn environments continue to take precedence, while omitted thread selections preserve the existing default behavior.

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

codex-rs/protocol/src/protocol.rs

@@ -567,6 +567,10 @@ pub enum Op {
         /// Updated personality preference.
         #[serde(skip_serializing_if = "Option::is_none")]
         personality: Option<Personality>,
+
+        /// Updated sticky environment selections for future turns.
+        #[serde(default, skip_serializing_if = "Option::is_none")]
+        environments: Option<Vec<TurnEnvironmentSelection>>,
     },
 
     /// Approve a command execution