Make ambient temperature configurable and wire it everywhere it was hardcoded

brewpi.py passed a literal 20 as theta_amb to Pot, and
temp_controller_smith.py's two internal Pot models (and every demo's
Pot/TempController instantiation) silently relied on Pot's default of
20 instead. Add a top-level ambient_temperature key to
config.json(.templ/.sim), thread it through brewpi.py into both the
real plant and the Smith predictor's models via a new theta_amb param
on TempController, and make the demos pass it explicitly too.

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

scripts/demos/plant/demo_pot.py

@@ -5,6 +5,7 @@ from components.plant.pot import Pot
 
 if __name__ == '__main__':
 	dt = 1.0
+	theta_amb = 20
 	power_on = 3500
 	steps_heat = 4000
 	steps_cool = 40000
@@ -18,7 +19,7 @@ if __name__ == '__main__':
 		"gain"		: 1.0
 	}
 
-	pot = Pot(dt, pot_params)
+	pot = Pot(dt, pot_params, theta_amb)
 
 	_t = np.empty(0)
 	_temp = np.empty(0)