docs(agents): clarify sibling data0/src resolution in combined workspace

[sskyy]

Summary

While setting up the development environment for the combined axiijs workspace (axii + data0 + axle checked out side by side), I found the ## Cursor Cloud specific instructions note about data0 resolution was inaccurate for that layout:

• It said "no sibling checkout is present here" — but when the repos are checked out together, a sibling ../data0/src is present, and npx vitest run resolves data0 from source.
• The sibling alias is actually applied in vitest.config.ts (it is commented out in vite.config.ts), so the previous file reference was misleading.

This is a docs-only change (one line in AGENTS.md); no code behavior changes.

Testing

Environment was fully validated during setup (unchanged by this doc edit):

• npm run build — ✅ emits dist/
• npx vitest run — ✅ 672 browser tests pass (real chromium via Playwright)
• npx vitest run --config vitest.node.config.ts — ✅ 6 node tests pass