Add optional Xquik research provider

[kriptoburak]

Summary

Adds Xquik as an optional research provider for the existing enhanced research tools.

The current SocialData.tools integration remains the first provider when SOCIALDATA_API_KEY is configured. When it is not configured, the server can use XQUIK_API_KEY against Xquik's public API contract for tweet search, user profiles, user tweets, followers, and following data.

What Changed

• Added XquikClient to adapt Xquik tweet and user responses into the existing research handler shape.
• Updated the research client factory to select SocialData.tools first, otherwise Xquik.
• Added Xquik environment variables to .env.example.
• Updated README and CLAUDE.md provider setup docs.
• Kept existing MCP tool names and handlers unchanged.

Validation

• npm run build
• Missing-key JSON-RPC smoke test for advancedTweetSearch
• Mocked Xquik adapter smoke test for search response mapping and contract header
• git diff --check
• Changed-file URL check: public docs links returned 200; API base URLs were skipped because endpoint roots may require a path or auth