docs: add Cursor Cloud development environment instructions

[dcroote]

Adds Cloud Agent development environment documentation and a reusable UI smoke test.

Changes

• AGENTS.md — Cursor Cloud instructions for tests, app, and UI verification
• scripts/verify-ui.mjs — Headless Playwright check for both tabs:
  • Single mAb Analysis: loads Trastuzumab example → Analyze
  • Multiple mAb Analysis: loads 4-sequence example CSV → Cluster & Analyze
• package.json — pnpm verify-ui script

Verified

• node test.js — 607 passed
• node scripts/verify-ui.mjs — Single + Multiple tabs pass; screenshots saved

Single mAb analysis
Multiple mAb analysis

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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