Waitlist → Mission Control early access with grandfathered pricing

[brianorwhatever]

From docs/business-plan.md §7 Phase 1 (weeks 5–6). Converts scarcity into activation using the waitlist that already exists (convex/waitlist.ts).

• Gate advanced Mission Control features (run dashboards, retention controls) behind early access
• Invite waitlist in batches; early users get grandfathered Agent Pro pricing (depends on Agent tier, Define Agent tier pricing + create Stripe products #196)
• Onboarding email sequence: quickstart → first run → invite your team
• Measure batch-by-batch activation to tune onboarding before opening fully