docs: add AGENTS.md with Cursor Cloud development instructions

[rishabhyadavv]

Adds AGENTS.md with development environment instructions for Cursor Cloud agents.

What's included

• Key development commands reference (lint, typecheck, test, build)
• Environment notes: Node.js v18 requirement, Yarn 3.6.1 Berry, example app limitations in Cloud VM
• web-secure-encryption polyfill tip for Node.js testing
• Pre-commit hooks note (lefthook)

Verification

All development commands verified working:

dev_environment_verification.log

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