Decide and execute: context-monitor.sh --drain path + its timer are undocumented, possibly dead

context-monitor.sh still carries the --drain code path and a dedicated systemd service+timer (context-venus-monitor-drain, created 2026-07-31) that runs it. If the path is dead, both the code and the units should go; if it is live, its purpose needs stating in the script header, because nothing currently documents what a drain run is for. Verify which before removing anything - the timer is enabled and running today, so this is a read-then-decide item, not a delete.