feat: add custom headers to send-email and send-batch-emails

[mayankbohradev]

Summary

• Resend's send and batch APIs already accept a headers object for custom MIME headers (for example List-Unsubscribe and X-Entity-Ref-ID).
• send-email and send-batch-emails did not expose that field, so agents could not set unsubscribe or threading headers through the MCP.
• Add optional headers (string key/value map) to both tools and pass it through to the Resend SDK payload.
• Update the README features list to mention custom headers (and idempotency keys, which are already on main).

Why this matters

Without custom headers, MCP users cannot:

• add one-click unsubscribe headers required for many bulk/transactional flows
• set X-Entity-Ref-ID to control Gmail threading

This matches Resend's documented Node SDK usage:

await resend.emails.send({
  from: 'Acme <onboarding@resend.dev>',
  to: ['delivered@resend.dev'],
  subject: 'hello world',
  text: 'it works!',
  headers: {
    'List-Unsubscribe': '<https://example.com/unsubscribe>',
    'X-Entity-Ref-ID': 'order-123',
  },
});

Test plan

• pnpm exec biome check --write .
• pnpm lint
• pnpm test (112 passing, including new headers cases)
• pnpm build