feat: automate Winget package submission

[rianjs]

Summary

Automates Winget package submission using wingetcreate to create PRs to microsoft/winget-pkgs.

Changes

New: Manual Workflow (.github/workflows/winget-publish.yml)

For initial submission and retries:

1. Go to Actions → "Publish to Winget"
2. Enter version (e.g., 0.9.9)
3. Workflow creates PR to microsoft/winget-pkgs

Updated: Release Workflow (.github/workflows/release.yml)

Adds winget job that runs after GoReleaser to automatically submit new releases.

Documentation Updates

• CLAUDE.md - Updated Winget entry in packaging section
• packaging/winget/README.md - Added automation section

Required Setup

1. Create GitHub PAT with public_repo scope at https://github.com/settings/tokens
2. Add as org/repo secret named WINGET_GITHUB_TOKEN

How It Works

Unlike Chocolatey (direct publish), Winget uses wingetcreate --submit which:

1. Downloads the release artifacts
2. Computes SHA256 checksums
3. Generates manifest files
4. Creates a PR to microsoft/winget-pkgs
5. Microsoft's automation validates and merges

Closes #77