an open challenge episode is pinned to its registration terms

Agent-decided, under AGENTS.md §6. A global change to threshold, evaluator or protocol applies to everything registered afterwards; applying it to an already open challenge requires an explicit transfer to a successor episode. Without this the moving-goalpost attack succeeds with nothing erased, nothing rejected and no reason ever promoted, and with every other premise satisfied. Pinning binds the episode rather than the process, so substantive standards remain revisable.