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/src/lib.rs

@@ -15,6 +15,7 @@ pub use cli::Command;
 pub use cli::ReviewArgs;
 use codex_app_server_client::DEFAULT_IN_PROCESS_CHANNEL_CAPACITY;
 use codex_app_server_client::EnvironmentManager;
+use codex_app_server_client::EnvironmentManagerArgs;
 use codex_app_server_client::ExecServerRuntimePaths;
 use codex_app_server_client::InProcessAppServerClient;
 use codex_app_server_client::InProcessClientStartArgs;
@@ -497,8 +498,8 @@ pub async fn run_main(cli: Cli, arg0_paths: Arg0DispatchPaths) -> anyhow::Result
         cloud_requirements: run_cloud_requirements,
         feedback: CodexFeedback::new(),
         log_db: None,
-        environment_manager: std::sync::Arc::new(EnvironmentManager::from_env_with_runtime_paths(
-            Some(local_runtime_paths),
+        environment_manager: std::sync::Arc::new(EnvironmentManager::new(
+            EnvironmentManagerArgs::from_env(local_runtime_paths),
         )),
         config_warnings,
         session_source: SessionSource::Exec,