Add Xquik skill

[kriptoburak]

Summary

• Add a Community/xquik skill for setting up Xquik REST, OpenAPI, webhook, and MCP workflows.
• Include public entry points, auth handling, secret hygiene, and safe request patterns.

Validation

• Per-skill validator passed for Community/xquik.
• Repo validator exited 0 with existing upstream warnings unrelated to this skill.
• Staged diff whitespace check passed.
• Xquik OpenAPI and MCP manifest links resolved.

[kriptoburak]

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