docs: Third-party integration guide (consume API/contracts)

[joelpeace48-cell]

Background & current state

Integrators must reverse-engineer how to consume Trivela (REST via backend/openapi.yaml, contracts via bindings). There's no single integration guide, which blocks the "build on Trivela" goal.

Goal

Author docs/INTEGRATION_GUIDE.md: a complete guide for third parties to integrate via the SDK/API/contracts, with runnable examples.

Outline

• Auth (API key + SEP-10 NEW-011), SDK install (NEW-085), campaign read/write, allowlist + proofs, webhooks (signature verification), contract calls via bindings, pagination, error handling, rate limits.
• A worked end-to-end example app referenced from CI (so it stays correct).

Task breakdown

• Write the guide with runnable snippets.
• Add an example integration exercised in CI.
• Cross-link from README + docs index; docs CI (prettier/link check).

Acceptance criteria

• A sample integration is reproducible by following the doc; example runs in CI.

Verification

• CI runs the example; maintainer review; link check.

Difficulty: medium · Effort: M · documentation + dx