Document MCP composition and connected-app credentials

[roshanjonah]

Summary

• Documents hosted and local MCP product-line composition across Management, Mailbox, and Sending surfaces.
• Documents connected-app credentials, short-lived access tokens, rotating refresh tokens, credential filtering, and disconnect/re-authorise behaviour.
• Updates the Sending OpenAPI snapshot and generated Postman collection from the corrected Sending API contract.

Verification

• npm run postman:check
• npx mint validate
• git diff --check
• rg -n "SendMux" openapi-sending.json postman/sendmux-sending.postman_collection.json guides changelog.mdx || true

Notes

• This PR must merge before the app/control-plane PR deploy because the app changelog/docs gate requires pushed docs for user-facing changes.

[mintlify]

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Project	Status	Preview	Updated (UTC)
sendmux	🟢 Ready	View Preview	Jun 10, 2026, 11:49 AM

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