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/pid/demo_temp_controller.py

@@ -31,13 +31,14 @@ if __name__ == '__main__':
 	}
 
 	dt = 1.0
+	theta_amb = 20
 	k_noise = 0.001
 
 	temp_ist = 0
 	temp_soll = 20
 	heatrate_soll = 1.25
 	ctrl = TempController(dt, ctrl_params)
-	plant = Pot(dt, plant_params)
+	plant = Pot(dt, plant_params, theta_amb)
 	_y = np.empty(0)
 	_fb = np.empty(0)
 	_t = np.empty(0)