fix: book on a plain yes, never push after a no, and always hang up

[davidtilser]

Live testing surfaced four voice agent problems:

• Double confirmation: the prompt asked for "explicit agreement" before accept_slot, which the agent read as needing a second yes. A clear yes is now explicit agreement; it books immediately and never re-asks.
• Pushing after a decline: the agent offered alternatives unprompted. A no is now final — decline_slot, one sentence of thanks, end. Other times come up only if the customer asks.
• Unreliable hangup: end_call was instructed for both outcomes but only enforced on the decline path. The instruction now covers both, and the web client ends the session itself if the agent keeps talking after a booked or declined result.
• Past times: the demo date was hardcoded to a fixed day, so offered slots were often in the past. It is now tomorrow in the shop's timezone, with the real UTC offset for that date, and a database left from an earlier demo day reseeds onto the new date. The dashboard reads the same DEMO_DATE.

npm run voice:update pushes the new rules to an existing agent; superseded sentences from earlier prompt versions are stripped first so an updated agent never carries contradictory instructions.