Define capability commerce terms and execution contract #546

Part of mctlhq/.github#173 (Agent Commerce). Roadmap ID: commerce-contract. Required.

Dependencies: mctlhq/mctl-agents#242, #266, #373 and #428 (all completed at audit).

Problem

Capability descriptors and model usage records cannot currently express an immutable customer charge or payer-bound invocation. Author a reviewed cross-repository contract extending ADR-017 and reusing ADR-012's attribution rules.

Acceptance

• Specify canonical capability/provider references, principal/tenant/payer binding, currency and integer amounts, versioned prices, immutable quotes and expiry.
• Define free/per_call/per_successful_call and every failure case in the plan; execution success, metered usage, payment status and settlement remain distinct.
• Define idempotency, provider capability requirements, durable state transitions, migration/backward compatibility and recovery ownership.
• Define PaymentAuthorizer authorize/capture/void/status contract, adapter feature negotiation and future refund records without a blockchain dependency.
• Define required discovery/read fields and authorization; signed payment proof is never an authorization grant. No production schema/API contract is implied by the planning document before C1 review.

This is a roadmap planning issue. It does not authorize real money, wallets, payouts, marketplace behavior or production payment rails, and it carries no implementation-triggering label. Design source: docs/plans/agent-commerce.md.