Add AGENTS.md with Cursor Cloud dev environment setup notes

[jarugupj]

Summary

Sets up the Cloud Agent development environment for this cua monorepo and documents the durable, non-obvious setup learnings in a new AGENTS.md.

• Confirmed Node >=22.19.0 via nvm (VM default /exec-daemon/node is v22.14.0); shell activation persists through ~/.bashrc.
• Verified npm install, full workspace build, typecheck, and unit tests across all packages.
• Built the ptywright native binding (Zig 0.15.2 + ghostty) so the CLI TUI fixture tests run.
• Documented external-key requirements (KERNEL_API_KEY + a provider key) and the ptywright build/test caveats.

Update script

npm install (minimal; Node selection handled via ~/.bashrc, native ptywright build is optional/dev-only and documented in AGENTS.md).

Testing

• npm run build (full workspace incl. native binding) — pass
• npx tsc -b (typecheck) — pass
• npm test --workspace @onkernel/cua-ai — 88 passed
• npm test --workspace @onkernel/cua-agent -- --exclude "**/*.live.test.ts" — 32 passed
• PTYWRIGHT_REQUIRED=1 npm test --workspace @onkernel/cua-cli — 42 passed (incl. 5 TUI fixtures)
• npx tsx packages/cli/src/cli.ts --help / models — runs

Live browser e2e (-p task) requires KERNEL_API_KEY + a provider key, which are not set in this environment.

cua_env_setup.log

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