Add explicit thread environment selection

Add thread/start environment_id plumbing and explicit local/remote selection support while preserving the current default environment behavior. Include focused exec-server and app-server coverage for the new thread environment selection paths.

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

codex-rs/core/src/codex.rs

@@ -430,6 +430,7 @@ pub(crate) struct CodexSpawnArgs {
     pub(crate) auth_manager: Arc<AuthManager>,
     pub(crate) models_manager: Arc<ModelsManager>,
     pub(crate) environment_manager: Arc<EnvironmentManager>,
+    pub(crate) environment_id: Option<String>,
     pub(crate) skills_manager: Arc<SkillsManager>,
     pub(crate) plugins_manager: Arc<PluginsManager>,
     pub(crate) mcp_manager: Arc<McpManager>,
@@ -483,6 +484,7 @@ impl Codex {
             auth_manager,
             models_manager,
             environment_manager,
+            environment_id,
             skills_manager,
             plugins_manager,
             mcp_manager,
@@ -503,7 +505,7 @@ impl Codex {
         let (tx_event, rx_event) = async_channel::unbounded();
 
         let environment = environment_manager
-            .current()
+            .environment(environment_id.as_deref())
             .await
             .map_err(|err| CodexErr::Fatal(format!("failed to create environment: {err}")))?;
         let fs = environment