Add cached environment manager for exec server URL (#15785)

Add environment manager that is a singleton and is created early in
app-server (before skill manager, before config loading).

Use an environment variable to point to a running exec server.

codex-rs/app-server/src/lib.rs

@@ -40,6 +40,7 @@ use codex_core::ExecPolicyError;
 use codex_core::check_execpolicy_for_warnings;
 use codex_core::config_loader::ConfigLoadError;
 use codex_core::config_loader::TextRange as CoreTextRange;
+use codex_exec_server::EnvironmentManager;
 use codex_feedback::CodexFeedback;
 use codex_protocol::protocol::SessionSource;
 use codex_state::log_db;
@@ -355,6 +356,7 @@ pub async fn run_main_with_transport(
     session_source: SessionSource,
     auth: AppServerWebsocketAuthSettings,
 ) -> IoResult<()> {
+    let environment_manager = Arc::new(EnvironmentManager::from_env());
     let (transport_event_tx, mut transport_event_rx) =
         mpsc::channel::<TransportEvent>(CHANNEL_CAPACITY);
     let (outgoing_tx, mut outgoing_rx) = mpsc::channel::<OutgoingEnvelope>(CHANNEL_CAPACITY);
@@ -620,6 +622,7 @@ pub async fn run_main_with_transport(
             outgoing: outgoing_message_sender,
             arg0_paths,
             config: Arc::new(config),
+            environment_manager,
             cli_overrides,
             loader_overrides,
             cloud_requirements: cloud_requirements.clone(),