[Feature] Rewrite welcome agent system prompt for conversational onboarding #595

Summary

Rewrite the welcome agent's system prompt to create a natural, human-toned conversational onboarding experience that actively helps users connect skills.

Problem

The current prompt (src/openhuman/agent/agents/welcome/prompt.md) has structural issues:

• First turn is "silent tool calls only" — creates delay before any visible response
• Tone guidelines exist but the overall conversation feels template-driven, not natural
• Agent doesn't persistently try to get user to connect skills
• No clear strategy for graceful fallback if user refuses all connections
• Agent can complete onboarding too quickly without real engagement

Solution

New prompt should define a multi-turn conversational onboarding with these characteristics:

Tone:

• Human, casual, friendly — not corporate
• "hey" not "Hello and welcome to OpenHuman"
• Like a helpful friend setting up your new phone, not a customer service bot

Flow:

1. First message: concise greeting + what's connected/not connected (from injected state) + what the platform can do
2. Offer to help connect Gmail (primary target) — generate auth link via composio_authorize
3. If user connects, celebrate and offer next skill
4. If user hesitates, explain why Gmail matters (inbox management, scheduling, etc.) — try again naturally
5. Try 2-3 times across the conversation, not in a row
6. Showcase app capabilities naturally woven into the conversation
7. Only call complete_onboarding after genuine multi-turn engagement
8. If user declines everything, gracefully wrap up and explain how to connect later

What NOT to do:

• No "silent first turn" — first response IS the greeting
• No corporate speak or excessive enthusiasm
• No forcing — be persistent but respectful
• No completing onboarding after just 1 exchange

Acceptance criteria

• New prompt — Rewritten prompt.md with conversational onboarding flow
• Human tone — Casual, friendly, not corporate throughout
• Gmail priority — Agent tries to get at least Gmail connected, offers link in chat
• Capability showcase — Agent naturally shows what the platform can do
• Persistence — Tries 2-3 times if user hesitates, then gracefully moves on
• Completion guard — complete_onboarding only called after meaningful conversation (3+ exchanges)
• No silent turn — First response IS the visible greeting
• agent.toml updated — If any config changes needed (iterations, temperature, etc.)

Related

• Files: src/openhuman/agent/agents/welcome/prompt.md, src/openhuman/agent/agents/welcome/agent.toml
• Depends on: [Bug] isOnboarded (chat_onboarding_completed) flag set too early #591, [Feature] Speed up LLM-generated welcome message and give agent template context #592, [Feature] Inject connection state into every user message during onboarding #593, [Feature] Welcome agent offers skill auth links directly in chat #594
• Part of: Conversational onboarding agent epic

Parent: #599 — Conversational onboarding agent