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/src/session/tests.rs

@@ -1643,7 +1643,10 @@ async fn fork_startup_context_then_first_turn_diff_snapshot() -> anyhow::Result<
         .thread_manager
         .fork_thread(
             usize::MAX,
-            fork_config,
+            fork_config.clone(),
+            std::sync::Arc::new(codex_thread_store::LocalThreadStore::new(
+                codex_rollout::RolloutConfig::from_view(&fork_config),
+            )),
             rollout_path,
             /*persist_extended_history*/ false,
             /*parent_trace*/ None,