Ensure that when a visible fallback PR has been merged (e.g., outside
Orca with a deleted head branch), it is still accepted and refreshed by
branch lookup instead of being discarded as an implicit merged PR.
* Add `acceptMergedFallbackPR` option to GitHub branch lookups
* Enable this option during manual and background refreshes of fallback PRs
* Plumb the new option through preload APIs, IPC handlers, and RPC protocols