Polish README for open-source release

[bingran-you]

Summary

• Replace the long ecosystem-style README with a shorter open-source project README focused on env0 runtime contracts.
• Keep the main mock-service screenshot collage and remove the extra dashboard/devhub screenshots.
• Remove Hugging Face references, private/internal repo references, and nonessential external links.
• Ground service metadata and validation guidance in config.toml and existing docs.

Validation

• scripts/smoke_dev.sh
• python3 devhub/app.py --render-once
• README local-link / forbidden-reference / ASCII scan