从 Resend 迁移

I am moving an email integration from Resend to Bird. Route through it with me.
1. Check what you already have before setting anything up. If Bird's MCP server is connected, use its tools. If the Bird CLI is installed and signed in, use that. Either one is enough, and every step below is an action you take with whichever you have. Only if neither is present, follow https://bird.com/docs/ai/set-up-your-agent.md to set one up and sign me in. Every Bird docs page serves Markdown at its own URL with `.md` appended, so fetch that rather than the HTML.
2. Read https://bird.com/docs/guides/email/migrate/resend.md for the payload, suppression and event mapping, and https://bird.com/docs/guides/email/migrate.md for the order the steps go in.
3. Find and list my Resend usage in this repository before you change anything: the POST /emails and batch call sites and any SDK wrappers around them, the webhook handler and the URL it is registered at, and every domain I send from.
4. Register each of those sending domains with Bird and give me the DNS records to publish, following https://bird.com/docs/guides/email/sending-domains.md. Leave every DNS record my current provider uses exactly as it is: Bird's records are published alongside them and both providers authenticate side by side until I switch traffic. Publishing DNS affects mail for the whole domain, so show me the records and let me publish them.
5. Rebuild my suppression list and import it into Bird before any production traffic goes through Bird, so my first sends do not reach addresses that already bounced or complained. Resend publishes no suppression export, so there is no endpoint that returns this list: derive it from whatever bounce and complaint events I have stored from my webhook, from the dashboard's Emails view, and from contacts marked unsubscribed in any Audience I use. That means the list can be incomplete without either of us noticing, so show me the list you built and tell me which of those sources each address came from before you import anything. The Bird import takes one address per request and is idempotent, so a partial re-run is safe. https://bird.com/docs/guides/email/suppressions.md has the reason taxonomy.
6. Port the send call and the webhook handler using the mapping tables on the provider page. Verification is a header change rather than a rewrite here: Resend signs with Svix and Bird signs per Standard Webhooks, which is the same HMAC construction with the svix-* headers renamed to webhook-*, so keep my verifier and rename what it reads. The event shape does change: Resend's events are scoped to a message and Bird's are scoped to a recipient, so a send to three recipients yields three outcomes rather than one. See https://bird.com/docs/guides/webhooks.md and https://bird.com/docs/guides/email/events.md.
7. Run my whole integration against Bird's mail sandbox before any production traffic, following https://bird.com/docs/guides/email/testing-sandbox.md. Sandbox sends run the real pipeline without reaching an inbox or touching my sending reputation.
8. Stop and ask me wherever a step needs a decision. Do not point production traffic at Bird until I have seen the sandbox results and replied with the words cut over to Bird. Retiring the Resend path is a separate step that comes later: ask me again and wait for me to reply with the words retire the Resend path. A reply that agrees without naming what it is authorising is not authorisation. Finish by telling me what is left that only a person can do.