Add Gmail SMTP email backend

send_email: auto-resolve bare employee name to email

Fix register_guest confirmation loop with Gemini native-audio

Shorten do_nothing rejection to plain ASCII (was crashing Gemini 1011)

Fix duplicate notification emails on lingering-visitor + do_nothing spam

Direct send_email block to re-fire register_guest with the same name

Initial release: MethdAI Receptionist v1.0