Open automated PR to microsoft/winget-pkgs on stable tag

Parent: #121

Release workflow opens a PR to [microsoft/winget-pkgs](https://github.com/microsoft/winget-pkgs) on every stable tag, using `wingetcreate` or `komac` to generate the manifest update from the new release artifact (__W2__).

## Definition of done

- Workflow job runs on push to a stable tag (Nerdbank.GitVersioning stable ref — i.e. `main`).
- Uses `wingetcreate update` (default choice; reconsider `komac` if friction emerges).
- PR opened against `microsoft/winget-pkgs` from a maintainer's GitHub account (bot account or PAT — decide during implementation).
- PR passes upstream automated validation (`Validation - Installer`).
- Failure to open / failing validation surfaces in the release workflow output, not silent.