docs: mention Agent Skills open standard and Anthropic announcement

[marc0olo]

Summary

• Distinguishes the two specs the page was conflating:
  • Agent Skills — the open standard defining the SKILL.md format
  • Agent Skills Discovery RFC — the discovery protocol (.well-known/skills/, index.json)
• Credits Anthropic as publisher of the open standard (December 2025) with a link to the engineering blog post
• Notes the cross-platform portability intent of the standard

Sync recommendation

informed by dfinity/icskills — README.md, skills/*/SKILL.md