Add intro explaining what Agent Skills are and link to the two skill
files (`create-mcp-app`, `migrate-oai-app`). Reorganize headings and add
"Invoke a Skill" and "Test Your App" sections. Update `README.md` link
text and add tip to quickstart suggesting agent skills as an alternative.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>