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/realtime_conversation.rs

@@ -1623,6 +1623,7 @@ async fn conversation_startup_context_current_thread_selects_many_turns_by_budge
         .thread_manager
         .resume_thread_with_history(
             test.config.clone(),
+            codex_core::thread_store_from_config(&test.config),
             InitialHistory::Forked(history),
             auth_manager_from_auth(CodexAuth::from_api_key("dummy")),
             /*persist_extended_history*/ false,