Add ThreadManager sample crate (#20141)

Summary:
- Add codex-thread-manager-sample, a one-shot binary that starts a
ThreadManager thread, submits a prompt, and prints the final assistant
output.
- Pass ThreadStore into ThreadManager::new and expose
thread_store_from_config for existing callsites.
- Build the sample Config directly with only --model and prompt inputs.

Verification:
- just fmt
- cargo check -p codex-thread-manager-sample -p codex-app-server -p
codex-mcp-server
- git diff --check

Tests: Not run per request.

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

@@ -5,6 +5,7 @@ use codex_core::NewThread;
 use codex_core::Prompt;
 use codex_core::ResponseEvent;
 use codex_core::ThreadManager;
+use codex_core::thread_store_from_config;
 use codex_features::Feature;
 use codex_login::AuthManager;
 use codex_login::CodexAuth;
@@ -1114,7 +1115,7 @@ async fn prefers_apikey_when_config_prefers_apikey_even_with_chatgpt_tokens() {
         /*analytics_events_client*/ None,
     );
     let NewThread { thread: codex, .. } = thread_manager
-        .start_thread(config)
+        .start_thread(config.clone(), thread_store_from_config(&config))
         .await
         .expect("create new conversation");