Read the final user/assistant message from session transcripts and display
it in the picker alongside the session update time. Allows quick previews
of what's in each session without opening it.
Enable meaningful session titles via priority-based derivation:
1. displayName (user-set)
2. subject (group name)
3. First user message (truncated to 60 chars)
4. sessionId prefix + date fallback
Opt-in via includeDerivedTitles param to avoid perf impact on
regular listing. Reads only first 10 lines of transcript files.
Closes#1161
