Add founding agency offer

[im-anishraj]

Summary

• add a public founding agency offer section to the landing page
• document the manual referral loop, fraud controls, and future automated billing-credit plan
• link the follow-up referral billing-credit implementation issue (P2: Implement referral billing credits #26)
• update launch docs, README, and roadmap status

Validation

• npm run typecheck
• npm run lint
• npm test
• npm run build
• browser smoke: /#founding-offer rendered the offer/referral section with no console errors or horizontal overflow

Closes #13