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-store/src/lib.rs

@@ -19,6 +19,7 @@ pub use in_memory::InMemoryThreadStoreCalls;
 pub use live_thread::LiveThread;
 pub use live_thread::LiveThreadInitGuard;
 pub use local::LocalThreadStore;
+pub use local::LocalThreadStoreConfig;
 pub use remote::RemoteThreadStore;
 pub use store::ThreadStore;
 pub use types::AppendThreadItemsParams;
@@ -37,5 +38,6 @@ pub use types::StoredThreadHistory;
 pub use types::ThreadEventPersistenceMode;
 pub use types::ThreadMetadataPatch;
 pub use types::ThreadPage;
+pub use types::ThreadPersistenceMetadata;
 pub use types::ThreadSortKey;
 pub use types::UpdateThreadMetadataParams;