Support multiple managed environments (#18401)

## Summary
- refactor EnvironmentManager to own keyed environments with
default/local lookup helpers
- keep remote exec-server client creation lazy until exec/fs use
- preserve disabled agent environment access separately from internal
local environment access

## Validation
- not run (per Codex worktree instruction to avoid tests/builds unless
requested)

---------

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

codex-rs/exec-server/src/remote_process.rs

@@ -9,7 +9,7 @@ use crate::ExecProcess;
 use crate::ExecProcessEventReceiver;
 use crate::ExecServerError;
 use crate::StartedExecProcess;
-use crate::client::ExecServerClient;
+use crate::client::LazyRemoteExecServerClient;
 use crate::client::Session;
 use crate::protocol::ExecParams;
 use crate::protocol::ReadResponse;
@@ -17,7 +17,7 @@ use crate::protocol::WriteResponse;
 
 #[derive(Clone)]
 pub(crate) struct RemoteProcess {
-    client: ExecServerClient,
+    client: LazyRemoteExecServerClient,
 }
 
 struct RemoteExecProcess {
@@ -25,7 +25,7 @@ struct RemoteExecProcess {
 }
 
 impl RemoteProcess {
-    pub(crate) fn new(client: ExecServerClient) -> Self {
+    pub(crate) fn new(client: LazyRemoteExecServerClient) -> Self {
         trace!("remote process new");
         Self { client }
     }
@@ -35,8 +35,9 @@ impl RemoteProcess {
 impl ExecBackend for RemoteProcess {
     async fn start(&self, params: ExecParams) -> Result<StartedExecProcess, ExecServerError> {
         let process_id = params.process_id.clone();
-        let session = self.client.register_session(&process_id).await?;
-        if let Err(err) = self.client.exec(params).await {
+        let client = self.client.get().await?;
+        let session = client.register_session(&process_id).await?;
+        if let Err(err) = client.exec(params).await {
             session.unregister().await;
             return Err(err);
         }