Add AGENTS.md with Cursor Cloud development environment setup

[NeroAgent]

Related Issue

Development environment setup for Cursor Cloud agents.

Description

Adds AGENTS.md with Cursor Cloud specific instructions for developing in this codebase, including:

• Quick reference for all key commands (install, build, lint, test, dev)
• Non-obvious caveats discovered during setup (Ubuntu 24.04 package naming, protobuf generation prerequisites, pre-commit hooks)

Test Procedure

Full development environment verified:

• npm run install:all — dependencies installed successfully
• npm run protos — protobuf generation completed
• npm run lint — 1383 files checked, no errors
• npm run test:unit — 922 tests passing
• npm run test:webview — 133 tests passing
• npm run cli:test — 337 tests passing
• npm run compile — extension compiled successfully
• npm run build:webview — webview built successfully
• npm run dev — watch mode running with 0 TypeScript errors

Type of Change

• 📚 Documentation update

Pre-flight Checklist

• Changes are limited to a single feature, bugfix or chore (split larger changes into separate PRs)
• Tests are passing (npm test) and code is formatted and linted (npm run format && npm run lint)
• I have created a changeset using npm run changeset (required for user-facing changes)
• I have reviewed contributor guidelines

Additional Notes

Documentation-only change — no changeset needed.