ci: handle updated current-main release PR heads

[zclawz]

Fixes the release workflow failure seen in #1037 / run 29268398422.

The failed run checked out PR head d2b4d84, a merge commit created when the release branch had been updated against main before squash merge. Its tree matched current main, but the workflow tried to publish the merge commit itself and failed the reachability guard.

This keeps the existing empty-release-commit behavior and adds a current-main fallback: when the PR head is a merge commit whose tree equals origin/main, publish origin/main as the release commit.

[zclawz]

Working this failure from release PR #1037 / run 29268398422. The failing payload checked out a merge-style release head (d2b4d84) whose tree matched current main, but the workflow tried to publish that merge commit and failed the reachability guard. This PR keeps the normal empty release commit path and adds a current-main fallback for that updated-release-branch shape. Locally checked the resolver against d2b4d84; it resolves to current main (b8bf1b3) and passes reachability.