从 MailerSend 迁移

I am moving an email integration from MailerSend 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/mailersend.md for the payload, suppression and webhook mapping, and https://bird.com/docs/guides/email/migrate.md for the order the steps go in.
3. Find and list my MailerSend usage in this repository before you change anything: calls to /v1/email and any SDK wrappers around them, whether I send with template_id and personalization or with html, the webhook handler and the URL it is registered at, and every domain I send from. Tell me the list before you edit anything.
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 MailerSend 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. Export my suppressions and import them into Bird before any production traffic goes through Bird, so my first sends do not reach addresses that already bounced or complained. MailerSend keeps five lists under /v1/suppressions: hard bounces, spam complaints, unsubscribes and the blocklist are the four to carry over. Do NOT import the On Hold list: it is a temporary 72-hour hold that MailerSend clears by itself, and importing it would suppress those addresses permanently. 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. Both providers sign with HMAC-SHA256, so this is a re-implementation rather than new code: MailerSend sends one Signature header, Bird follows the Standard Webhooks scheme with webhook-id, webhook-timestamp and webhook-signature, and the signed string is constructed differently. Keep the constant-time comparison. 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 MailerSend path is a separate step that comes later: ask me again and wait for me to reply with the words retire the MailerSend 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.