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/core/tests/suite/client.rs

@@ -1103,9 +1103,7 @@ async fn prefers_apikey_when_config_prefers_apikey_even_with_chatgpt_tokens() {
                 .features
                 .enabled(Feature::DefaultModeRequestUserInput),
         },
-        Arc::new(codex_exec_server::EnvironmentManager::new(
-            /*exec_server_url*/ None,
-        )),
+        Arc::new(codex_exec_server::EnvironmentManager::default_for_tests()),
         /*analytics_events_client*/ None,
     );
     let NewThread { thread: codex, .. } = thread_manager

codex-rs/core/tests/suite/skills.rs

@@ -5,6 +5,7 @@ use anyhow::Result;
 use codex_core::ThreadManager;
 use codex_exec_server::CreateDirectoryOptions;
 use codex_exec_server::EnvironmentManager;
+use codex_exec_server::ExecServerRuntimePaths;
 use codex_exec_server::ExecutorFileSystem;
 use codex_login::CodexAuth;
 use codex_models_manager::collaboration_mode_presets::CollaborationModesConfig;
@@ -234,7 +235,15 @@ async fn list_skills_skips_cwd_roots_when_environment_disabled() -> Result<()> {
         codex_core::test_support::auth_manager_from_auth(CodexAuth::from_api_key("dummy")),
         SessionSource::Exec,
         CollaborationModesConfig::default(),
-        Arc::new(EnvironmentManager::new(Some("none".to_string()))),
+        Arc::new(EnvironmentManager::new(
+            codex_exec_server::EnvironmentManagerArgs {
+                exec_server_url: Some("none".to_string()),
+                local_runtime_paths: ExecServerRuntimePaths::new(
+                    std::env::current_exe()?,
+                    /*codex_linux_sandbox_exe*/ None,
+                )?,
+            },
+        )),
         /*analytics_events_client*/ None,
     );
     let new_thread = thread_manager.start_thread(config.clone()).await?;