Add: 402-first machine payments (price-before-work tool purchases)

[bettergraininfo-rgb]

Summary

Adds a new pattern file: patterns/402-first-machine-payments.md (category: Orchestration & Control).

The pattern: using HTTP 402 as a quote-before-work negotiation primitive so autonomous agents can buy capabilities mid-task with no accounts or API keys. The server returns a machine-readable payment challenge before doing any work; the agent checks the quoted price against a hard budget/price-cap, signs payment, and the provider verifies settlement on-chain before performing the work. Covers the roles, control points (budget guards), receipts-as-ledger, a human-in-the-loop storefront variant, and honest trade-offs (wallet cold-start, chain latency, refund awkwardness).

Pattern-first note: the file is written to teach the pattern generically, with the x402 protocol credited as the originator in based_on/source. It is not a product announcement — the reference implementation link appears once, in References, alongside the spec and RFC.

Schema: front matter follows SCHEMA.md (required fields present; status emerging; category from the enum). I could not run bun run build:data locally (no bun in my environment), so the auto-generated README/site sections will need a refresh — happy to push that regeneration if the CI artifact doesn't cover it.

Validation: file matches TEMPLATE.md section structure (Problem / Solution / Evidence / How to use it / Trade-offs / References).

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