1. fetch_open_threads: replace N+1 loop (2 queries per thread) with a
single query using correlated subqueries for note_count and started_by.
2. extract_key_decisions: track consumed notes so the same note is not
matched to multiple events, preventing duplicate decision entries.
3. build_timeline_excerpt_from_pipeline: log tracing::warn on seed/collect
failures instead of silently returning empty timeline.
