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.toml

@@ -87,6 +87,7 @@ members = [
     "state",
     "terminal-detection",
     "test-binary-support",
+    "thread-store",
     "codex-experimental-api-macros",
     "plugin",
 ]