feat(welcome-agent): add discovery phase to understand user intent before checklist #994

Description

Follow-up from #885. The welcome agent prompt currently jumps straight into a fixed setup checklist. It should first have a discovery phase — asking the user what brought them to OpenHuman, what tools they use daily, and what they're trying to accomplish — before guiding them through setup.

Current behavior

The agent walks through a fixed, linear checklist regardless of who the user is.

Expected behavior

Before starting the checklist, the welcome agent should:

1. Ask what brought the user to OpenHuman
2. Ask about their daily workflow and tools (Gmail, Slack, Notion, etc.)
3. Ask what's annoying them about their current setup
4. Build a mental model of the user to personalize the rest of the conversation

This context should then inform which checklist items to emphasize and which integrations to suggest first.