Payments Sandbox discovery (B1-B3)

Simulated Stripe-shaped payments for agent practice - no real money, not affiliated with any provider. Resources: /customers (deterministic from seed), /payment_intents (+/confirm), /charges/{id}, /refunds, /events (poll feed). Lifecycle: requires_payment_method -> requires_confirmation -> (requires_action for pm_card_3ds_required) -> succeeded | failed; the fixed test token IS the outcome. Idempotency-Key enforced on POSTs (same key + same body replays with X-Idempotent-Replay; different body 409s). Per-caller state, 24h idle TTL. Same auth/rate limits/trajectory capture as /agent/v1.