fix(mdm): run git-ai directly from the launchers using bg start --retry-secs

macOS names a LaunchAgent after its executable, so the sh -c retry wrapper
made the login item and the 'can run in the background' notice say "sh",
and a Managed Login Items profile could not match it by the git-ai signing
team. Per-user agents now run the git-ai binary directly with
--retry-secs 10 and log through StandardOut/ErrorPath; only --system without
--bin keeps a shell to resolve each user's home. The Windows launcher drops
its loop for the same flag. Linux keeps a one-line sh exec wrapper because
systemd rejects executable paths containing quotes or parentheses, but its
retry loop is gone too.

The e2e drivers now hold the daemon lock while triggering the login start to
prove --retry-secs rides out the race, and keep the always-failing binary
check for the failure path.

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