ThreadStore interface (#17659)

Introduce a ThreadStore interface for mediating access to the filesystem
(rollout jsonl files + sqlite db) based thread storage.

In later PRs we'll move the existing fs code behind a "local"
implementation of this ThreadStore interface.

This PR should be a no-op behaviorally, it only introduces the
interface.

codex-rs/thread-store/src/lib.rs

@@ -0,0 +1,32 @@
+//! Storage-neutral thread persistence interfaces.
+//!
+//! Application code should treat [`codex_protocol::ThreadId`] as the only durable thread handle.
+//! Implementations are responsible for resolving that id to local rollout files, RPC requests, or
+//! any other backing store.
+
+mod error;
+mod recorder;
+mod store;
+mod types;
+
+pub use error::ThreadStoreError;
+pub use error::ThreadStoreResult;
+pub use recorder::ThreadRecorder;
+pub use store::ThreadStore;
+pub use types::AppendThreadItemsParams;
+pub use types::ArchiveThreadParams;
+pub use types::CreateThreadParams;
+pub use types::GitInfoPatch;
+pub use types::ListThreadsParams;
+pub use types::LoadThreadHistoryParams;
+pub use types::OptionalStringPatch;
+pub use types::ReadThreadParams;
+pub use types::ResumeThreadRecorderParams;
+pub use types::SetThreadNameParams;
+pub use types::StoredThread;
+pub use types::StoredThreadHistory;
+pub use types::ThreadEventPersistenceMode;
+pub use types::ThreadMetadataPatch;
+pub use types::ThreadPage;
+pub use types::ThreadSortKey;
+pub use types::UpdateThreadMetadataParams;