Refactor: extract core, prepare for org split

[masonlet]

Restructures the codebase from a single Vercel-coupled implementation into core/, resend/, nodemailer/, and vercel/, mirroring the future repo split under the contact-api org.

• Extract platform-agnostic CORS evaluation out of the Vercel handler.
• Extract handleContact into a platform-agnostic core handler
• Move provider initialization out of config.ts into each provider's own module
• Restructure src/ and api/ into core/, resend/, nodemailer/, vercel/
• Split README into per-repo docs, rewrite root README as temporary index

Behaviour stays the same: all tests pass and coverage carried over/extended for the new provider factory functions.