feat(daemon): bg start --retry-secs for login-time lock races

bg shutdown returns before the old daemon process releases daemon.lock, so a
login start that fires in that window (fast logout/login, self-update) fails
with 'lock held' and nothing retries until the next checkpoint. The MDM login
launchers currently wrap bg start in a shell retry loop, which makes macOS
report the login item as "sh". Give bg start a bounded retry window instead
so the launchers can run the binary directly.

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