Add Xquik social signals skill

[kriptoburak]

Summary

• add xquik-social-signals under ruflo-market-data
• document bounded X signal ingestion into market-data
• keep writes, monitors, webhooks, and account-affecting actions behind explicit approval

Validation

• frontmatter audit for ruflo-market-data
• ruflo-market-data smoke script, 11 passed
• staged diff whitespace and public-safety scan
• Xquik docs and OpenAPI links returned 200