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/exec-server/src/remote_process.rs

@@ -1,5 +1,6 @@
 use async_trait::async_trait;
 use tokio::sync::broadcast;
+use tracing::trace;
 
 use crate::ExecProcess;
 use crate::ExecServerClient;
@@ -19,6 +20,7 @@ pub(crate) struct RemoteProcess {
 
 impl RemoteProcess {
     pub(crate) fn new(client: ExecServerClient) -> Self {
+        trace!("remote process new");
         Self { client }
     }
 }
@@ -26,10 +28,12 @@ impl RemoteProcess {
 #[async_trait]
 impl ExecProcess for RemoteProcess {
     async fn start(&self, params: ExecParams) -> Result<ExecResponse, ExecServerError> {
+        trace!("remote process start");
         self.client.exec(params).await
     }
 
     async fn read(&self, params: ReadParams) -> Result<ReadResponse, ExecServerError> {
+        trace!("remote process read");
         self.client.read(params).await
     }
 
@@ -38,14 +42,17 @@ impl ExecProcess for RemoteProcess {
         process_id: &str,
         chunk: Vec<u8>,
     ) -> Result<WriteResponse, ExecServerError> {
+        trace!("remote process write");
         self.client.write(process_id, chunk).await
     }
 
     async fn terminate(&self, process_id: &str) -> Result<TerminateResponse, ExecServerError> {
+        trace!("remote process terminate");
         self.client.terminate(process_id).await
     }
 
     fn subscribe_events(&self) -> broadcast::Receiver<ExecServerEvent> {
+        trace!("remote process subscribe_events");
         self.client.event_receiver()
     }
 }