fix: Simplify fallback value on error in skip merge queue action

[Mrtenz]

I merged #191 a bit too quickly, Cursor left a comment about a bug afterwards 😅.

The failure handler step writes up-to-date=false to $GITHUB_OUTPUT, but it doesn't have an id. The action's output is defined to reference steps.up-to-date.outputs.up-to-date, which only captures output from the step with id: up-to-date. When previous steps fail, the failure handler's output goes nowhere, leaving the action's output empty instead of "false".

I realised we can just add a fallback to the output directly instead of using a separate step.

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Note

Ensures the composite action always outputs up-to-date=false on errors by adding an output fallback and deleting the separate failure-handling step.

• GitHub Action (.github/actions/check-skip-merge-queue/action.yml):
  • Output: Set outputs.up-to-date to steps.up-to-date.outputs.up-to-date || 'false' to guarantee a false default on failures.
  • Cleanup: Remove the extra failure handler step that previously echoed up-to-date=false.

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