Add ESPHome firmware templates, GND pour notes, v2 lessons learned

- firmware/: config + secrets templates committed, flashed config git-ignored
- README: document the GND pour on F.Cu/B.Cu and the module keep-out rule areas
- README: correct the calibration button wiring (S8 bCal_in, not an MCU GPIO)
- README: v2 list - dedicated 5 V power input, decoupling, mounting holes,
  I2C pull-ups, schematic annotation, RoHS finish, project name, antenna keep-out

.gitignore

@@ -9,6 +9,11 @@ _restore_backup_*/
 *.gbr.bak
 *.drl.bak
 
+# Firmware credentials - never commit live keys
+firmware/esp-sensair.yaml
+firmware/secrets.yaml
+**/secrets.yaml
+
 # OS / editor noise
 .DS_Store
 *~