feat: add xquik social data skill

[kriptoburak]

Summary

• add xquik-social-data as a read-first Xquik REST API and MCP skill
• register it in the full and marketing marketplaces
• add README install, example, download, and structure entries

Duplicate Check

• No default-branch Xquik entry found
• No all-state Xquik PR or issue found before pushing
• Existing Hermes PR is a different provider surface

Validation

• python3 -m json.tool .claude-plugin/marketplace.json
• python3 -m json.tool .claude-plugin/marketplace-marketing.json
• ./scripts/build-skills.sh xquik-social-data
• git diff --check
• Xquik API docs, MCP docs, OpenAPI schema, and MCP manifest returned HTTP 200 or valid JSON