release: publish MDM login-start scripts as release assets

Copy mdm/*/install-login-start.* into the release as
git-ai-login-start-{macos,linux,windows}.{sh,ps1}, cover them in SHA256SUMS
(the existing release/git-ai-* globs already attest and upload them), and
list them in the release notes so MDM docs can point at
releases/latest/download/.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>