Add a first-touch attribution tag to the two signup CTAs so package-driven
signups are measurable on vatnode:
- README "get one / sign up free" links → /login?ref=mcp-readme
- validate_vat_number missing-key output → /login?ref=mcp-tool

Copy unchanged otherwise. Patch bump for release.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>