docs: add Cursor Cloud environment setup notes

[SteveShani]

• Ready for review
• Follows CONTRIBUTING rules
• Reviewed by Snyk internal team

What does this PR do?

Adds a ## Cursor Cloud specific instructions section to AGENTS.md capturing the non-obvious caveats discovered while setting up this repo in a Cursor Cloud Agent VM:

• FORCE_COLOR is required for npm run test:unit — without it the 4 display tests in test/lib/display.spec.ts fail because their fixtures embed ANSI color escapes and chalk strips colors when stdout is not a TTY.
• Docker Hub pulls are blocked by the VM's network egress (blob storage on S3/Cloudflare CDN); Docker official images must be pulled from the public.ecr.aws/docker/library/* mirror. The Docker engine is installed (docker-in-docker via fuse-overlayfs) but not auto-started.
• npm run test:system (and the sibling snyk-docker-pull tests) can't pull their target images here and rely on CI.
• A quick offline end-to-end smoke recipe for scan() against a saved archive.
• A workspace-wide note on which sibling repos need private NPM_TOKEN / GOPRIVATE GitHub credentials.

Where should the reviewer start?

AGENTS.md → ## Cursor Cloud specific instructions.

How should this be manually tested?

FORCE_COLOR=3 npm run test:unit → 873/873 pass. Documentation-only change.

Any background context you want to provide?

Produced during automated dev-environment setup. No source/test code changed.

What are the relevant tickets?

N/A

Screenshots

N/A — docs only.