[codex] Add rollout-backed thread content search (#23519)

## Summary
- add experimental `thread/search` for local rollout-backed thread
search using `rg` over JSONL rollouts
- return search-specific result rows with optional previews instead of
storing preview data on `StoredThread` or ordinary `Thread` responses
- keep `thread/list` separate from full-content search and document the
new app-server surface

## Testing
- `cargo test -p codex-app-server-protocol`
- `cargo test -p codex-app-server
thread_search_returns_content_and_title_matches -- --nocapture`

codex-rs/thread-store/src/local/mod.rs

@@ -4,6 +4,7 @@ mod helpers;
 mod list_threads;
 mod live_writer;
 mod read_thread;
+mod search_threads;
 mod unarchive_thread;
 mod update_thread_metadata;
 
@@ -28,9 +29,11 @@ use crate::LoadThreadHistoryParams;
 use crate::ReadThreadByRolloutPathParams;
 use crate::ReadThreadParams;
 use crate::ResumeThreadParams;
+use crate::SearchThreadsParams;
 use crate::StoredThread;
 use crate::StoredThreadHistory;
 use crate::ThreadPage;
+use crate::ThreadSearchPage;
 use crate::ThreadStore;
 use crate::ThreadStoreError;
 use crate::ThreadStoreResult;
@@ -261,6 +264,13 @@ impl ThreadStore for LocalThreadStore {
         list_threads::list_threads(self, params).await
     }
 
+    async fn search_threads(
+        &self,
+        params: SearchThreadsParams,
+    ) -> ThreadStoreResult<ThreadSearchPage> {
+        search_threads::search_threads(self, params).await
+    }
+
     async fn update_thread_metadata(
         &self,
         params: UpdateThreadMetadataParams,