Document Cursor Cloud setup caveats

[saggiyogesh]

Summary

• Add AGENTS.md with Cursor Cloud-specific development environment notes discovered during setup.
• Document API-only operation when KVM/base images are unavailable, local-auth API key bootstrap, and VM-runtime-related flags for API checks.

Verification

• go mod download
• go vet ./...
• go test ./...
• go build ./cmd/server ./cmd/setup-net
• Started MongoDB and VoidRun API in development mode.
• Created an API key via local-auth bootstrap, restarted with normal auth, and called protected user/image/sandbox endpoints with X-API-Key.

Artifacts

• Setup checks log: /opt/cursor/artifacts/env_setup/voidrun-setup-checks.log
• API demo transcript: /opt/cursor/artifacts/env_setup/voidrun-api-demo.log