fix: stabilize autograder workflows from validated template snapshots

[accesswatch]

Ports validated autograder reliability fixes from learning-room-template PR snapshots (#5-#9) into source-of-truth learning-room workflows.\n\nIncludes:\n- watchdog workflow\n- update-or-create comment behavior\n- failure fallback paths\n- improved reliability under reruns and silent failures\n\nThis fixes root-cause drift by applying the changes at source before template sync.

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Automated validation by Learning Room Bot. Aria generated this review. Last updated: 2026-05-10T02:02:34.696Z
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