ci: add windows-amd64 and windows-arm64 to release matrix

[scott]

Motivation

The current release matrix covers darwin/{amd64,arm64} and linux/{amd64,arm64}. Windows is missing entirely, which blocks every tier-2 Windows packaging path (Scoop, winget, Chocolatey) and forces Windows users through WSL or source builds. Adding Windows to the matrix is a one-step change in .github/workflows/release.yml and is the prerequisite for the Tier-2 packaging PRs.

Acceptance criteria

1. The matrix in .github/workflows/release.yml gains two entries:
   • { target: windows-amd64, goos: windows, goarch: amd64 }
   • { target: windows-arm64, goos: windows, goarch: arm64 }
2. Output binaries for the new targets are suffixed .exe (so dist/gander-windows-amd64.exe, dist/gander-windows-arm64.exe).
3. Per-asset SHA256 sidecars are produced for the new targets with the matching naming (gander-windows-amd64.exe.sha256, etc.).
4. release.yml's files: list under softprops/action-gh-release@v3 is updated to include the four new files.
5. assetNameForRuntime in upgrade.go is reviewed; no change is expected (it should match gander-{goos}-{goarch}.exe for Windows automatically — verify, adjust only if necessary).
6. CI smoke check: a fresh tag (e.g. v0.0.0-rc.1) produces all four new artifacts and the resulting gander-windows-amd64.exe --help exits 0 under wine or a Windows runner spot-check.

Out of scope

• macOS code-signing / notarization (separate work).
• Final binary smoke-testing on each platform — the smoke check above is sufficient for this issue.