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/Cargo.lock

@@ -2824,6 +2824,17 @@ dependencies = [
  "tempfile",
 ]
 
+[[package]]
+name = "codex-thread-store"
+version = "0.0.0"
+dependencies = [
+ "async-trait",
+ "chrono",
+ "codex-protocol",
+ "serde",
+ "thiserror 2.0.18",
+]
+
 [[package]]
 name = "codex-tools"
 version = "0.0.0"