fix: handle concurrent push rejection in generate-leaderboard workflow

[Copilot]

When multiple workflow runs overlap (e.g. scheduled + push-triggered), the second git push gets rejected with fetch first because the remote has advanced since checkout.

Changes

• generate-leaderboard.yml: Add git pull --rebase --autostash origin main between git commit and git push to incorporate any remote commits before pushing. Includes explicit failure message if rebase hits unresolvable conflicts.

-          git commit -m "chore: update leaderboard data"
-          git push
+          git commit -m "chore: update leaderboard data"
+          git pull --rebase --autostash origin main || (echo "Rebase failed due to conflicts" && exit 1)
+          git push

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