ci(mdm): nightly follows the update channel and tolerates pre-retry releases

The daemon self-updates to whatever the 'latest' channel at
usegitai.com/worker/releases serves, which lags the newest GitHub release
while a release is being promoted; the auto-update scenario now waits for the
channel version instead. The held-lock phase is skipped when the installed
release predates bg start --retry-secs, so the nightly stays meaningful
between merging this stack and the next release.

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