[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/Cargo.lock

@@ -3744,6 +3744,7 @@ dependencies = [
  "async-trait",
  "chrono",
  "codex-git-utils",
+ "codex-install-context",
  "codex-protocol",
  "codex-rollout",
  "codex-state",