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/window_headers.rs

@@ -71,6 +71,7 @@ async fn window_id_advances_after_compact_persists_on_resume_and_resets_on_fork(
         .fork_thread(
             /*snapshot*/ 0usize,
             resumed.config.clone(),
+            codex_core::thread_store_from_config(&resumed.config),
             rollout_path,
             /*persist_extended_history*/ false,
             /*parent_trace*/ None,