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/mcp-server/src/codex_tool_runner.rs

@@ -13,6 +13,7 @@ use codex_core::CodexThread;
 use codex_core::NewThread;
 use codex_core::ThreadManager;
 use codex_core::config::Config as CodexConfig;
+use codex_core::thread_store_from_config;
 use codex_protocol::ThreadId;
 use codex_protocol::protocol::AgentMessageEvent;
 use codex_protocol::protocol::ApplyPatchApprovalRequestEvent;
@@ -68,7 +69,10 @@ pub async fn run_codex_tool_session(
         thread_id,
         thread,
         session_configured,
-    } = match thread_manager.start_thread(config).await {
+    } = match thread_manager
+        .start_thread(config.clone(), thread_store_from_config(&config))
+        .await
+    {
         Ok(res) => res,
         Err(e) => {
             let result = CallToolResult {