feat: use GoReleaser Pro for split/merge CGO builds

- Add goreleaser-key input for Pro license
- Use goreleaser-pro distribution for split/merge modes
- Split: goreleaser release --clean --split
- Merge: goreleaser continue --merge
- Pass GORELEASER_PRO secret to action

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.github/workflows/go-release-cgo.yaml

@@ -160,6 +160,7 @@ jobs:
           mode: split
           cgo-enabled: "1"
           github-token: ${{ secrets.GITHUB_TOKEN }}
+          goreleaser-key: ${{ secrets.GORELEASER_PRO }}
 
       - name: Upload artifacts
         uses: actions/upload-artifact@v4
@@ -199,6 +200,7 @@ jobs:
           version-tag: ${{ needs.version.outputs.version_tag }}
           mode: merge
           github-token: ${{ secrets.GITHUB_TOKEN }}
+          goreleaser-key: ${{ secrets.GORELEASER_PRO }}
 
       - name: Rolling release
         if: inputs.rolling-release-tag != ''