v1.20.0 — x402 in one tool: the agent-pays loop

Added — x402 in one tool + the pay-per-call spec

• chaingpt_x402_fetch (134 → 135) — the whole agent-pays loop, custody-free: fetch an x402-protected URL; 2xx returns the body; 402 decodes the PaymentRequirements and (given from) emits the UNSIGNED EIP-3009 typed data; after external signing, re-call with xPaymentHeader to complete the paid request. Prefers exact-scheme challenges on known networks, handles malformed 402 bodies gracefully, https-only.
• Pay-per-call spec for api.chaingpt.org (.plans/x402-pay-per-call-spec.md): 402 challenge shape, credit-mirroring pricing map, verify-then-settle-async with a replay cache, failure semantics, phased rollout. The goal: agents pay $0.01 USDC per call with zero signup — the first major AI API agents can pay autonomously. Agent-wallet AUTO-pay deliberately deferred to a policy extension (USDC caps are a new dimension; same fail-closed discipline required).
• skills/x402 teaches the one-tool fast path.

Tests

• Suite 393 → 399 (fetch orchestrator: 2xx, https-only, challenge decode, typed-data build, paid retry header, malformed 402).