salt.schedule.highstate_interval_hours could not express a sub-hour
cadence, so an operator who disables salt.auto_apply had no way back to
the legacy 15-minute highstate. Rename the setting to
highstate_interval_minutes (default 120, behavior unchanged) and enforce
a 15-minute floor in SOC.
Non-manager splay is now a quarter of the interval clamped to [5, 30]
minutes, so a short interval no longer gets jitter larger than itself; at
the 120-minute default it stays 1800s. The so-salt-minion-check restart
threshold keeps its interval-plus-one-hour grace, now in minute math.
Josh Patterson
b109ca4e9b