docs: link Harness and Agent Skills across READMEs

[denial123789]

Summary

• add a compact SandBase open-source stack section immediately after the CLI quick start
• link directly to SandBase Harness and SandBase Agent Skills
• keep English, Chinese, Japanese, Korean, Spanish, French, German, and Portuguese READMEs synchronized

This converts upcoming CLI directory exposure into direct discovery for the two related open-source repositories instead of leaving the projects disconnected.

Validation

• each of the eight READMEs contains exactly one Harness link and one Skills link
• npm test passed
• npm run build passed
• git diff --check passed