Add CI check to detect `main` → `new-resolver-config` merge conflicts

[smoparth]

Context

The new-resolver-config branch is a long-lived feature branch where multiple contributors will land PRs. As main progresses (new features, refactors, dependency updates), the feature branch can silently diverge, leading to painful merge conflicts discovered late when we try to merge the completed feature back to main.

Parent epic: #1254

Problem

There is no automated way to detect when main and new-resolver-config have diverged to the point where merging would produce conflicts. Without early detection, conflicts accumulate and become harder to resolve.

Acceptance criteria

• A GitHub Action runs automatically (on push to main, on a daily schedule, or both) and attempts a trial merge of main into new-resolver-config
• If the merge has conflicts, the Action clearly reports which files conflict
• If the merge is clean, no action is taken (or a previously opened alert is closed)
• The check does NOT actually push any merge commits — it is a dry-run detection only
• The team is notified when conflicts are detected (e.g., via an issue, PR comment, or workflow failure visible in the Actions tab)

Notes

• Use git merge --no-commit --no-ff in a temporary checkout for the dry-run
• Consider running on both schedule (daily) and push to main triggers
• Merge direction should be main → new-resolver-config (not rebase, since the branch is shared)
• This Action can be removed once new-resolver-config is merged back to main