chore(app-server) world-writable windows notification (#6880)

## Summary
On app-server startup, detect whether the experimental sandbox is
enabled, and send a notification .

**Note**
New conversations will not respect the feature because we [ignore cli
overrides in
NewConversation](a75321a64c/codex-rs/app-server/src/codex_message_processor.rs (L1237-L1252)).
However, this should be okay, since we don't actually use config for
this, we use a [global
variable](87cce88f48/codex-rs/core/src/safety.rs (L105-L110)).
We should carefully unwind this setup at some point.


## Testing
- [ ] In progress: testing locally

---------

Co-authored-by: jif-oai <jif@openai.com>

codex-rs/app-server/src/codex_message_processor.rs

@@ -111,6 +111,7 @@ use codex_core::config_loader::load_config_as_toml;
 use codex_core::default_client::get_codex_user_agent;
 use codex_core::exec::ExecParams;
 use codex_core::exec_env::create_env;
+use codex_core::features::Feature;
 use codex_core::find_conversation_path_by_id_str;
 use codex_core::get_platform_sandbox;
 use codex_core::git_info::git_diff_to_remote;
@@ -1249,7 +1250,17 @@ impl CodexMessageProcessor {
             ..Default::default()
         };
 
-        let config = match derive_config_from_params(overrides, cli_overrides).await {
+        // Persist windows sandbox feature.
+        // TODO: persist default config in general.
+        let mut cli_overrides = cli_overrides.unwrap_or_default();
+        if cfg!(target_os = "windows") && self.config.features.enabled(Feature::WindowsSandbox) {
+            cli_overrides.insert(
+                "features.enable_experimental_windows_sandbox".to_string(),
+                serde_json::json!(true),
+            );
+        }
+
+        let config = match derive_config_from_params(overrides, Some(cli_overrides)).await {
             Ok(config) => config,
             Err(err) => {
                 let error = JSONRPCErrorError {