GitHub Merge Queue bug reverts prior commits

This was a correctness incident, not a simple outage, which makes it more dangerous because the system could appear healthy while writing bad history.

• –The regression hit merge queue squash merges only when the group contained multiple PRs.
• –GitHub says the faulty code path came from merge base computation and was meant to be feature-flagged, but the gating was incomplete.
• –Automated monitoring missed it because the problem affected merge correctness rather than availability.
• –GitHub mitigated by reverting the change, force-deploying the fix, and sending targeted recovery guidance to affected repositories.
• –The practical lesson is that merge-queue systems need explicit multi-PR content-integrity checks, not just green checks and uptime signals.