User decision (2026-07-05): evolve from single-user to public multi-tenant SaaS.
Adds PRODUCT_DIRECTION.md: email linking generalises beyond Gmail (Microsoft
Graph + IMAP + always-available free-text fallback), SaaS platform wave
(onboarding, billing, quotas, per-tenant AI budget, rate limiting, outbox), and
resolves the .gsd "use next.js" override in favour of executing it (public SEO).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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