Auto-trigger rework when a PR becomes conflicting

The build pipeline now automatically detects merge conflicts on open PRs after any push to main. When a PR develops a conflict because an unrelated PR was merged, the pipeline queues rework without requiring a human nudge — as long as the 3-round rework budget has not been exhausted. The auto-rework budget counter is never touched by this automated trigger; only a @vello human nudge can reset it, preserving the runaway-loop guarantee.