P1: Include zip in SHA256SUMS.txt for releases

Problem

Release SHA256SUMS.txt currently omits cloudsqlctl-windows-x64.zip, even though the zip is published as a release asset.
This weakens release integrity checks for users/package managers (Scoop/winget/choco) that depend on the zip hash.

Acceptance criteria

Test plan

Run
pm run stage and confirm �rtifacts/SHA256SUMS.txt contains a line for cloudsqlctl-windows-x64.zip.

Plan

Update ools/stage-artifacts.ps1 to generate SHA256SUMS.txt after the zip is created and include both .exe and .zip entries.
Validate locally by running:
pm run package,
pm run installer,
pm run stage and confirming �rtifacts/SHA256SUMS.txt contains cloudsqlctl-windows-x64.zip.

Risk

Ensure checksum generation stays deterministic and filenames match uploaded release assets.

Done via #48
Merge commit: 0b85a1d

Verification

pm run stage now writes SHA256SUMS entries for cloudsqlctl.exe, cloudsqlctl-setup.exe, and cloudsqlctl-windows-x64.zip.