feat(@hanzo/commerce): extend client with checkout, coupons, referrals, affiliates

Add comprehensive commerce client methods:
- validateCoupon / applyCoupon (pre-checkout validation)
- createCheckoutSession (hosted checkout, returns checkoutUrl)
- tokenizeCard (S2S card tokenization without external SDK)
- referrals: createReferralProgram, getReferralLink, trackReferral, getReferralStats
- affiliates: registerAffiliate, getAffiliateStats, getAffiliateCommissions
- Backwards-compatible: commerceUrl still works, baseUrl preferred
- Default baseUrl: https://api.hanzo.ai
- Request timeout support (timeoutMs, default 15s)

Author: hanzo-dev