Make thread store process-scoped (#19474)

- Build one app-server process ThreadStore from startup config and share
it with ThreadManager and CodexMessageProcessor.
- Remove per-thread/fork store reconstruction so effective thread config
cannot switch the persistence backend.
- Add params to ThreadStore create/resume for specifying thread
metadata, since otherwise the metadata from store creation would be used
(incorrectly).

codex-rs/thread-manager-sample/src/main.rs

@@ -118,12 +118,13 @@ async fn run_main(arg0_paths: Arg0DispatchPaths) -> anyhow::Result<()> {
         SessionSource::Exec,
         environment_manager,
         /*analytics_events_client*/ None,
+        Arc::clone(&thread_store),
     );
 
     let NewThread {
         thread_id, thread, ..
     } = thread_manager
-        .start_thread(config, thread_store)
+        .start_thread(config)
         .await
         .context("start Codex thread")?;