Refactor config loading to use filesystem abstraction (#18209)

Initial pass propagating FileSystem through config loading.

codex-rs/core/src/agent/role.rs

@@ -18,6 +18,7 @@ use crate::config_loader::resolve_relative_paths_in_config_toml;
 use anyhow::anyhow;
 use codex_app_server_protocol::ConfigLayerSource;
 use codex_config::config_toml::ConfigToml;
+use codex_exec_server::LOCAL_FS;
 use std::collections::BTreeMap;
 use std::collections::BTreeSet;
 use std::path::Path;
@@ -168,6 +169,7 @@ mod reload {
         }
 
         let mut next_config = Config::load_config_with_layer_stack(
+            LOCAL_FS.as_ref(),
             merged_config,
             reload_overrides(config, preserve_current_provider),
             config.codex_home.clone(),