ci(mdm): per-PR end-to-end tests for the login-start scripts

Add scripts/mdm/test-login-start.{sh,ps1}, which install git-ai, register
the login start, simulate the login trigger, and assert the daemon comes up,
survives the launcher exiting, restarts itself on schedule while registered,
and stays healthy after a re-trigger. The lifecycle scenario runs per PR on
ubuntu, macOS, and Windows via .github/workflows/mdm-login-start.yml with a
locally built binary; the auto-update scenario is wired up for the nightly
workflow.

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