fix: stabilize grading workflows and add watchdog

[accesswatch]

Ports proven autograder reliability fixes into source-of-truth learning-room workflows:\n\n- update-or-create comment behavior for challenge autograders\n- failure fallback handling\n- watchdog workflow for silent runner failure detection\n\nThis prevents future drift and keeps learning-room-template resets deterministic via sync.

[github-actions]

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Automated validation by Learning Room Bot. Aria generated this review. Last updated: 2026-05-10T02:00:34.922Z
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[github-actions]

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Hi @accesswatch! Your PR has been automatically paired with @taylorarndt for peer review.

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[github-actions]

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