feat: add Windows code signing with SignPath (#76)

- Integrate SignPath.io for Windows artifact signing (MSI, NSIS)
- Add sign-windows job to release workflow
- Upload unsigned artifacts as temporary GitHub artifacts
- Sign artifacts via SignPath API before release upload
- Rename workflows: build.yml -> release.yml, build-artifacts.yml -> build.yml
- Update DEVELOPMENT.md with new workflow structure
- Add SignPath acknowledgment to README.md

Author: SuperKali