fix(updater): keep nightly staged updates replaceable
A cmux NEXT NIGHTLY can enter Sparkle's silent-install path when an older SUAutomaticallyUpdate preference remains set. Sparkle then owns the staged installer and stops the normal replaceable background-check path, so a restart can install the stale item while newer nightlies wait. This PR makes background mode own the staged reply by disabling that legacy silent-install preference when the updater is configured, preserving the scheduled check cadence and allowing the staged item to be replaced by the newest feed result.
The regression test seeds the persisted automatic-download preference, enables background mode, and verifies the silent path is disabled. Existing background-install tests continue to cover staged install and prompt behavior.
Validation:
git diff --check- Fleet package test:
cmux-ci run --class isolated --script scripts/ci/run-swift-testing-suites.sh --ref <head> --arg Packages/macOS/CmuxUpdater(huge-blue client is not authorized for the fleet controller; the PR's routed CI covers the same package) - Independent Codex correctness-first review: SHIP at
437e98b789be39ab3b25be5642eb658d51bfbffd
Closes the NIGHTLY symptom where SULastCheckTime stopped after staging and idle install never fired.
Agent session: huge-blue