Add fail-closed Agent Guild policy for x402 payments

[AgentTanuki]

Summary

• add opt-in --x402-policy agent-guild --x402-max-amount <atomic>
• wait for the authoritative 402, buy a short-lived AGPD-1 decision, and locally verify its Ed25519/JCS proof, issuer, freshness, thresholds, and exact payment tuple before signing
• persist the policy and ceiling across session recovery/restart, and refuse silent payment-mode changes
• isolate policy-approved retry signatures per async call so concurrent tool calls cannot consume each other's authorization

Safety properties

• existing --x402 behavior is unchanged unless the policy flag is supplied
• protected payments fail closed on network, decision, signature, issuer, freshness, threshold, field, resource-origin, or amount-ceiling mismatch
• the policy-decision purchase is separately pinned to exact/Base mainnet/USDC/the Agent Guild treasury/a $0.01 maximum; redirects are refused
• guarded mode disables speculative pricing-metadata signing and does not use the bridge's process-wide payment cache
• non-HTTP mcp:// payment resources remain an explicit fail-closed compatibility boundary because AGPD-1 currently requires an HTTP(S) resource

The policy decision is itself a paid x402 call, so the wallet needs Base USDC for the decision and protected tool call.

Validation

• TypeScript compile
• full unit suite: 1,023 passed
• focused policy/middleware/output suite: 203 passed
• ESLint: 0 errors (6 existing non-null-assertion warnings)
• Prettier
• generated CLI reference check
• viem bundle
• git diff --check

No live payments were made while implementing or testing this change.