add(core): managed config (#3868)

## Summary

- Factor `load_config_as_toml` into `core::config_loader` so config
loading is reusable across callers.
- Layer `~/.codex/config.toml`, optional `~/.codex/managed_config.toml`,
and macOS managed preferences (base64) with recursive table merging and
scoped threads per source.

## Config Flow

```
Managed prefs (macOS profile: com.openai.codex/config_toml_base64)
                               ▲
                               │
~/.codex/managed_config.toml   │  (optional file-based override)
                               ▲
                               │
                ~/.codex/config.toml (user-defined settings)
```

- The loader searches under the resolved `CODEX_HOME` directory
(defaults to `~/.codex`).
- Managed configs let administrators ship fleet-wide overrides via
device profiles which is useful for enforcing certain settings like
sandbox or approval defaults.
- For nested hash tables: overlays merge recursively. Child tables are
merged key-by-key, while scalar or array values replace the prior layer
entirely. This lets admins add or tweak individual fields without
clobbering unrelated user settings.

codex-rs/mcp-server/src/codex_tool_config.rs

@@ -132,7 +132,7 @@ pub(crate) fn create_tool_for_codex_tool_call_param() -> Tool {
 impl CodexToolCallParam {
     /// Returns the initial user prompt to start the Codex conversation and the
     /// effective Config object generated from the supplied parameters.
-    pub fn into_config(
+    pub async fn into_config(
         self,
         codex_linux_sandbox_exe: Option<PathBuf>,
     ) -> std::io::Result<(String, codex_core::config::Config)> {
@@ -172,7 +172,8 @@ impl CodexToolCallParam {
             .map(|(k, v)| (k, json_to_toml(v)))
             .collect();
 
-        let cfg = codex_core::config::Config::load_with_cli_overrides(cli_overrides, overrides)?;
+        let cfg =
+            codex_core::config::Config::load_with_cli_overrides(cli_overrides, overrides).await?;
 
         Ok((prompt, cfg))
     }