[codex] Add local thread store listing (#17824)

Builds on top of #17659 

Move the filesystem + sqlite thread listing-related operations inside of
a local ThreadStore implementation and call ThreadStore from the places
that used to perform these filesystem/sqlite operations.

This is the first of a series of PRs that will implement the rest of the
local ThreadStore.

Testing:
- added unit tests for the thread store implementation
- adjusted some unit tests in the realtime + personality packages whose
callsites changed. Specifically I'm trying to hide ThreadMetadata inside
of the local implementation and make ThreadMetadata a sqlite
implementation detail concern rather than a public interface, preferring
the more generate StoredThread interface instead
- added a corner case test for the personality migration package that
wasn't covered by the existing test suite
- adjust the behavior of searched thread listing to run the existing
local rollout repair/backfill pass _before_ querying SQLite results, so
callers using ThreadStore::list_threads do not miss matches after a
partial metadata warm-up

codex-rs/rollout/src/recorder.rs

@@ -279,6 +279,10 @@ impl RolloutRecorder {
         let codex_home = config.codex_home();
         let state_db_ctx = state_db::get_state_db(config).await;
 
+        // Search is the SQLite-optimized path and assumes a DB marked backfill-complete is
+        // actually populated enough to answer the query. If unmigrated rollout files still exist
+        // on disk, the repair path below may or may not run and catch them depending on whether
+        // SQLite already has another matching search hit.
         if search_term.is_some()
             && let Some(db_page) = state_db::list_threads_db(
                 state_db_ctx.as_deref(),