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>

config.json.templ

@@ -1,4 +1,5 @@
 {
+	"ambient_temperature": 20,
 	"Controller" : {
 	"dt": 1,
 	"sim_warp_factor": 10.0,