Add ambient temperature gate to heating period detection

Adds heating_min_ambient_c (default 15°C) to season config. When outdoor
temperature is at or above this threshold, SG-Ready is suppressed even if
the calendar month is within the heating season. Prevents unnecessary heat
pump boost activation on warm spring/autumn days.

Logic: heating active = in_heating_month AND ambient < threshold
Zero value (unset) disables the temperature gate (calendar-only, old behaviour).

New test: TestHeatingPeriodAmbientSuppression covers warm-day suppression,
cold-day pass-through, and summer month independence.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/engine/engine.go

@@ -107,7 +107,7 @@ func (e *Engine) Decide(state collector.SystemState, now time.Time, wwBoostC flo
 
 	soc := state.BatterySOC
 	gridW := state.GridPowerW // positive = import, negative = export
-	heatingPeriod := e.isHeatingPeriod(now)
+	heatingPeriod := e.isHeatingPeriod(now, state.AmbientTempC)
 	wwWindow := e.isWWWindow(now)
 	allowed := e.allowedConsumers(soc)
 
@@ -502,17 +502,37 @@ func (e *Engine) ApplyOverride(consumer Consumer, on bool, duration time.Duratio
 	)
 }
 
-// isHeatingPeriod returns true if the current month is within the heating season.
-func (e *Engine) isHeatingPeriod(now time.Time) bool {
+// isHeatingPeriod returns true if heating is appropriate given the current month
+// and outdoor temperature. If HeatingMinAmbientC is configured (> 0), ambient
+// temperatures above that threshold suppress SG-Ready even within the heating months.
+func (e *Engine) isHeatingPeriod(now time.Time, ambientC float64) bool {
 	month := int(now.Month())
 	start := e.cfg.Season.HeatingStartMonth
 	end := e.cfg.Season.HeatingEndMonth
 
 	// Handles wrap-around: e.g. October(10) to April(4)
+	var inMonth bool
 	if start > end {
-		return month >= start || month <= end
+		inMonth = month >= start || month <= end
+	} else {
+		inMonth = month >= start && month <= end
 	}
-	return month >= start && month <= end
+
+	if !inMonth {
+		return false
+	}
+
+	// Temperature override: warm day within heating months → not a heating day
+	threshold := e.cfg.Season.HeatingMinAmbientC
+	if threshold > 0 && ambientC >= threshold {
+		e.logger.Debug("heating period suppressed by ambient temperature",
+			"ambient_c", ambientC,
+			"threshold_c", threshold,
+		)
+		return false
+	}
+
+	return true
 }
 
 // minRuntime returns the minimum runtime for a consumer.