`Agent.Email`: curl-based signup flow for AI agents

Agents get a restricted inbox first, then unlock it through a human OTP reply. Useful as a product-design signal: agent-native onboarding needs CLI-first UX, rate limits, and human ownership.

• —The flow starts with curl; browser requests get HTML, agents get Markdown. Machine-readable onboarding is treated as the default surface.
• —Unclaimed inboxes can email only their owner, capped at 10 emails/day, with IP rate limits. Trust is scoped before expansion.
• —A human replies with an OTP to claim the agent, creating a 1:1 agent-to-human mapping. Many-to-one is the next obvious need.
• —CLI formatting and shorter message IDs changed after agents struggled with mixed delimiters and hallucinated long IDs. Agent UX differs from human UX.