Repo-switch safety needs to answer, synchronously on the UI thread,
"is any foreground work in flight right now?" so it can refuse a switch
that would run against a repo about to be swapped out. gocui already
tracks a task per OnWorker/Update for the test idle-listener; extend
that.
Tasks gain a background flag: background tasks (the ongoing routines
like auto-fetch, and the refreshes they trigger) don't count towards
busy, because their model writes are already guarded against a
concurrent switch by the repo generation. Add OnWorkerBackground,
UpdateBackground and UpdateContentOnlyBackground (plus the gui-layer
OnUIThreadBackground / OnUIThreadContentOnlyBackground / OnWorkerBackground
on IGuiCommon) so the few background call sites can opt in without
touching the hundreds of foreground callers.
TaskManager.hasBusyForegroundTaskExcept answers the query; Gui.Busy()
wraps it, excluding the event currently being processed (recorded as
currentTask) so a handler asking the question doesn't count itself.
Nothing gates on Busy() yet; this is the mechanism only.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
