correct agent_prompt: only use blastx for short nuc sequences

[rusalkaguy]

blastx will run out of memory at NCBI Blast, if you use it on even a small genome, like the example test_data/genome/poxvirus.NY_014.fna

Longer (genome) nuc queries must be run with megablast (close hits) or discontinuous megablast (medium homology) or blastn (low homology).

draft_configuration-agent_prompt.md just says to always use blastx for nucleotide.