Harden release workflow against immutable existing tag releases

[Copilot]

The Create Release GitHub Actions job failed on reruns for an existing tag because asset upload was attempted against an already-created immutable release (422 Cannot upload assets to an immutable release). This change makes the release path safe for repeated executions on the same tag.

• Root-cause alignment

  • softprops/action-gh-release was always invoked, even when a release for github.ref_name already existed.

• Workflow change

  • Added a pre-check step in .github/workflows/build.yml to detect whether the tag release already exists.
  • Gated the release creation/upload step to run only when no existing release is found.

• Behavioral impact

  • First run on a new tag still creates release + uploads assets.
  • Subsequent runs on the same tag skip upload instead of failing.

- name: Check if release already exists
  id: release_check
  env:
    GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
  run: |
    if gh release view "${{ github.ref_name }}" > /dev/null 2>&1; then
      echo "exists=true" >> "$GITHUB_OUTPUT"
    else
      echo "exists=false" >> "$GITHUB_OUTPUT"
    fi

- name: Create GitHub Release
  if: steps.release_check.outputs.exists != 'true'
  uses: softprops/action-gh-release@v1