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/code_mode.rs

@@ -2556,6 +2556,7 @@ async fn code_mode_can_call_hidden_dynamic_tools() -> Result<()> {
         .thread_manager
         .start_thread_with_tools(
             base_test.config.clone(),
+            codex_core::thread_store_from_config(&base_test.config),
             vec![DynamicToolSpec {
                 namespace: Some("codex_app".to_string()),
                 name: "hidden_dynamic_tool".to_string(),