Revert "ci/github-script/bot: skip mergeability checks temporarily"

Apparently any effects from this change haven’t shown up noticeably
in GitHub’s metrics, to the point where they’re not sure if it
was taking effect on the backend. Our contact is going to look at
getting something into the API response to help debug whether it’s
actually working or not, but agreed that we should just revert for
now. Since they have apparently reduced replica sync issues further
through other changes on their end, there shouldn’t be any urgent
need to make any changes here anyway.

This reverts commit 18b30c8ce1.

(cherry picked from commit 742b1081c5)

ci/github-script/bot.js

@@ -145,22 +145,10 @@ module.exports = async ({ github, context, core, dry }) => {
 
     // This API request is important for the merge-conflict label, because it triggers the
     // creation of a new test merge commit. This is needed to actually determine the state of a PR.
-    //
-    // NOTE (2025-12-15): Temporarily skipping mergeability checks here
-    // on GitHub’s request to measure the impact of the resulting ref
-    // writes on their internal metrics; merge conflicts resulting from
-    // changes to target branches will not have labels applied for the
-    // duration. The label should still be updated on pushes.
-    //
-    // TODO: Restore mergeability checks in some form after a few days
-    // or when we hear back from GitHub.
     const pull_request = (
       await github.rest.pulls.get({
         ...context.repo,
         pull_number,
-        // Undocumented parameter (as of 2025-12-15), added by GitHub
-        // for us; stability unclear.
-        skip_mergeability_checks: true,
       })
     ).data