docs: add real Coinbase webhook delivery and verification guide

[GsCommand]

Motivation

• Provide clear, actionable documentation for delivering real Coinbase CDP webhook events into the existing signed example endpoint POST /api/examples/coinbase-webhook so operators can perform live integrations.
• Explain required setup, expected endpoint behavior, and how to verify CommandLayer-signed receipts without exposing secrets or changing existing HMAC/signing code.

Description

• Added a new Real Coinbase delivery section to docs/integrations/coinbase-cdp-webhook-receipts.md that documents the end-to-end delivery path to POST https://www.commandlayer.org/api/examples/coinbase-webhook.
• Documented setup steps for Coinbase CDP (create project and webhook subscription), recommended event types (payments.transfers.completed or other payments.transfers.*), and the required Vercel environment variables COINBASE_WEBHOOK_SECRET, CL_RECEIPT_SIGNER_ID, CL_RECEIPT_SIGNING_KID, and RECEIPT_SIGNING_PRIVATE_KEY_PEM_B64, plus redeploy instructions.
• Added explicit expected endpoint responses for GET and unsigned POST, described the success response WEBHOOK_VERIFIED_AND_SIGNED, and added receipt verification instructions using POST https://www.commandlayer.org/api/verify with expected fields VERIFIED, hash_matches: true, and signature_valid: true.
• Added troubleshooting guidance for invalid_signature, stale_signature, signing_unavailable, and normalization_failed, and a concise Real delivery readiness checklist for operational validation; no code signing/HMAC logic was modified and no secrets or private keys were added to the repo.

Testing

• Ran unit/integration tests with npm test, which completed successfully with 95 passing tests.
• Attempted npm run check:links but the repository does not define a check:links script (run reported Missing script: "check:links").

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Codex Task

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Project	Deployment	Actions	Updated (UTC)
commandlayer-commandlayer-org	Ready	Preview, Comment	May 23, 2026 11:49pm
commandlayer-org	Ready	Preview, Comment	May 23, 2026 11:49pm
commandlayer-org111	Ready	Preview, Comment	May 23, 2026 11:49pm