Get the current overlay plan for the device: a time-ordered sequence of commands starting at startTime, each active for durationMinutes before the next begins. If no overlay plan is active, startTime is a sentinel value in the past and commands is empty.

Command kinds are device-type-specific: selfConsumption and idle apply to battery/inverter devices (charge from excess solar, or hold without charging or discharging); setpoint also applies to battery/inverter devices and targets a specific charge or discharge power in watts; heatWater is exclusive to heat-pump water heaters and has no meaning for batteries, inverters, or EV chargers.