docs: add Cursor Cloud dev environment setup notes to AGENTS.md

[AiRanthem]

Ⅰ. Describe what this PR does

Documents the Cursor Cloud development environment setup for this repo under a new ## Cursor Cloud specific instructions section in AGENTS.md. No product code is changed. The notes capture non-obvious, durable caveats discovered while bringing up the dev environment:

• A full official Go SDK (1.25+) must live at /usr/local/go; the auto-downloaded golang.org/toolchain modules in this environment are stripped and lack the covdata tool, which breaks make test's coverage on no-test packages.
• make test uses envtest (real kube-apiserver + etcd, no external cluster needed).
• Build components by package dir (single-file go build main.go fails on executeCABindings); the gateway plugin needs CGO c-shared.
• A full Kubernetes node (kind/minikube/k3d) cannot run here because cgroup v2 memory/io/hugetlb controllers can't be delegated to child cgroups. Run the agent-sandbox-controller against an envtest control plane instead (webhooks disabled, metrics off).

Ⅱ. Does this pull request fix one issue?

NONE

Ⅲ. Describe how to verify it

Verified the full dev workflow in the cloud VM:

• go build ./cmd/... — all 5 components build (controller, manager, traffic-extension, and the CGO sandbox-gateway.so).
• make test — full pkg/... unit suite passes with coverage (after installing the full Go SDK).
• make lint — 0 issues.
• Ran bin/agent-sandbox-controller against a live envtest control plane with the 8 CRDs installed, applied examples/code_interpreter/sandboxset.yaml (replicas=2) and scaled to 3; the operator reconciled it into child Sandbox and Pod objects with the correct SandboxSet → Sandbox → Pod owner-reference chain.

Ⅳ. Special notes for reviews

Only AGENTS.md is modified. The update script for future cloud agents is go mod download; the Go SDK, envtest binaries, Docker, kind, and kubectl are provisioned in the VM snapshot.