Add Xquik plugin

[kriptoburak]

Summary

• Add a partner-built Xquik plugin package.
• Configure the authenticated Xquik MCP server with an XQUIK_API_KEY environment variable.
• Register the plugin in the marketplace and include one X research skill.

Validation

• Confirmed no existing Xquik plugin, PR, or recent issue.
• Confirmed root Apache-2.0 license and plugin file structure.
• Validated marketplace, plugin, and MCP JSON with jq.
• Checked public Xquik docs, website, GitHub repo, and npm package metadata.
• Ran diff whitespace validation.

[kriptoburak]

Closing because the repo scope guard only accepts non-member PRs for marketplace entries backed by already-live plugins, and this branch adds plugin files. The policy blocker is valid.