Tune sub-query count (5-8) after benchmark

[smodee]

The spec sets 5-8 sub-queries per question. After the first benchmark run, check:

• Are redundant axes wasting Tavily API calls?
• Do some question types need fewer/more sub-queries?
• Is the total cap of 60 pre-dedup results appropriate?

Files: bioscancast/stages/search_stage/query_decomposition.py, pipeline.py