docs: add AGENTS.md with Cursor Cloud dev environment instructions

[O1ahmad]

Summary

Adds AGENTS.md with development environment guidance for Cloud Agents working in this Docker-first Web3 containers monorepo.

What's documented

• How to start Docker in Cloud Agent VMs (fuse-overlayfs storage driver)
• Per-service compose workflow (network=local, port conflict notes)
• CI-equivalent smoke test commands (build + compose up/down)
• Example hello-world RPC checks for avalanchego

Environment verification performed

• Installed Docker CE + Compose plugin
• Built 0labs/avalanchego:1.10.5 image
• Ran compose up/down smoke test
• Verified JSON-RPC endpoints:
  • info.getNodeVersion → avalanche/1.9.8 (image build arg vs bundled binary)
  • eth_blockNumber on C-chain → 0x0

VM update script

Minimal pip refresh for CI tooling: yq (no application package dependencies in this repo).