Implement ACP checkout adapter

[pozylon]

Summary

• add ACP 2026-04-17 checkout session REST endpoints for Express and Fastify
• add OpenAI flat JSONL product feed, ACP well-known metadata, and signed order webhooks
• add Stripe Shared Payment Token payment adapter and register it in the full plugin preset
• document required ACP env/provider setup

Verification

• npm run build
• npm run lint
• npm run test

Notes: the first test attempt hit a local MongoMemoryServer port conflict on 4011; after clearing the stale mongod process, the full test suite passed. A subsequent run also hit a flaky pre-existing schedule performance threshold once, then the final full run passed.