Hardcoded --partition=main in every stage script would break on any
cluster without a partition of that exact name. Moved it to a
SLURM_PARTITION config variable, passed as a CLI flag (omitted
entirely if unset) instead of baked into each script's #SBATCH
header, matching how NOTIFY_EMAIL is already handled.

Also documents the hard SLURM+scrontab dependency in the README, and
cleans up stale docs/ references and a couple of leftover artifacts
from the previous edit pass.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>