docs: correct data0 sibling-checkout note in Cursor Cloud instructions

[sskyy]

Summary

While setting up the Cloud Agent development environment, I found one now-inaccurate statement in AGENTS.md's ## Cursor Cloud specific instructions.

The old note claimed data0 has "no sibling checkout present here" and referenced vite.config.ts. In this multi-repo workspace the sibling ../data0 repo is checked out, and it's vitest.config.ts (not vite.config.ts) that aliases data0 to ../data0/src when the sibling exists. Updated the bullet to describe the actual resolution behavior for both the test suite (sibling ../data0/src) and the production build (npm-installed data0 for externalized code + bundled types).

No code changes — documentation only.

Verification

• ✅ npm install + npx playwright install chromium + npm run build (emits dist/)
• ✅ npx vitest run — 398 browser tests pass (Vitest + Playwright chromium, headless)
• ✅ npx vitest run --config vitest.node.config.ts — 6 node tests pass
• The sibling axii-benchmark app runs against the freshly built dist/axii.js and a benchmark scenario executes end to end.