CONTRIBUTING.md says packages/sdk has no test script, but package.json already defines one

[vjuliaife]

Description:
CONTRIBUTING.md's Testing table states "packages/sdk has no dedicated test script yet" and instructs contributors to type-check it and exercise it via API integration tests instead. But packages/sdk/package.json already defines "test": "tsx --test src/compatibility.test.ts". The doc is stale and hides a faster feedback loop from contributors.

Acceptance Criteria:

• CONTRIBUTING.md's Testing table is updated to reference npm run test --workspace=packages/sdk
• Confirm the existing compatibility.test.ts actually runs and passes via the documented command
• Remove the now-inaccurate guidance about needing to exercise the SDK only through API integration tests
• Cross-reference README/docs for any other place repeating the stale claim

Relevant Files:

• CONTRIBUTING.md — Testing table row for SDK is stale
• packages/sdk/package.json — already has "test": "tsx --test src/compatibility.test.ts"