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/tui/src/app/tests.rs

@@ -1592,6 +1592,7 @@ fn update_memory_settings_updates_current_thread_memory_mode() -> Result<()> {
         let (mut app, _app_event_rx, _op_rx) = Box::pin(make_test_app_with_channels()).await;
         let codex_home = tempdir()?;
         app.config.codex_home = codex_home.path().to_path_buf().abs();
+        app.config.sqlite_home = codex_home.path().to_path_buf();
         // Seed the previous setting so this test exercises the thread-mode update path.
         app.config.memories.generate_memories = true;