Add Xquik backend and repair action examples

[kriptoburak]

Summary

• Add opt-in backend: xquik support for connected Xquik accounts.
• Keep x-api as the default and validate backend-specific credentials.
• Document the new inputs and rebuild the bundled action.
• Repair existing action metadata and workflow examples.

Independent repository fixes

• Replace the invalid YAML null input default with an empty string.
• Replace deprecated ::set-output syntax with $GITHUB_OUTPUT.
• Upgrade example checkout steps from v2 to v4.
• Repair the broken x.com README link.

Validation

• npm test -- --runInBand
• npm run build
• Parsed action.yaml with js-yaml
• Verified the repaired link and removed deprecated example syntax
• git diff --check

[kriptoburak]

Repair commit 668852c is public.

The repair corrects the action input default, deprecated output syntax, outdated checkout examples, and broken README link. The full 7-test suite passes, the bundled action rebuilds cleanly, action.yaml parses, and the complete diff passes whitespace and disclosure checks.